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Decode UTF-8 bytes as Latin-1 characters

I have a string that I receive from a third party app and I would like to display it correctly in any language using C# on my Windows Surface.
Due to incorrect encoding, a piece of my string looks like this in Farsi (Persian-Arabic):
مدل-رنگ-موی-جدید-5-436x500
whereas it should look like this:
مدل-رنگ-موی-جدید-5-436x500
This link convert this correctly:
http://www.ltg.ed.ac.uk/~richard/utf-8.html
How I can do it in c#?

It is very hard to tell exactly what is going on from the description of your question. We would all be much better off if you provided us with an example of what is happening using a single character instead of a whole string, and if you chose an example character which does not belong to some exotic character set, for example the bullet character (u2022) or something like that.
Anyhow, what is probably happening is this:
The letter "ر" is represented in UTF-8 as a byte sequence of D8 B1, but what you see is "ر", and that's because in UTF-16 Ø is u00D8 and ± is u00B1. So, the incoming text was originally in UTF-8, but in the process of importing it to a dotNet Unicode String in your application it was incorrectly interpreted as being in some 8-bit character set such as ANSI or Latin-1. That's why you now have a Unicode String which appears to contain garbage.
However, the process of converting 8-bit characters to Unicode is for the most part not destructive, so all of the information is still there, that's why the UTF-8 tool that you linked to can still kind of make sense out of it.
What you need to do is convert the string back to an array of ANSI (or Latin-1, whatever) bytes, and then re-construct the string the right way, which is a conversion of UTF-8 to Unicode.
I cannot easily reproduce your situation, so here are some things to try:
byte[] bytes = System.Text.Encoding.Ansi.GetBytes( garbledUnicodeString );
followed by
string properUnicodeString = System.Text.Encoding.UTF8.GetString( bytes );

UTF-8 Encoding and Decoding in c#

I Searched for " How to Encode the data in utf-8 format". Regarding this I got the best result is following:
UTF8Encoding utf8 = new UTF8Encoding();
String unicodeString = "ABCD";
// Encode the string.
Byte[] encodedBytes = utf8.GetBytes(unicodeString);
// Decode bytes back to string.
String decodedString = utf8.GetString(encodedBytes);
But the Problem is when I see the encoded data I found that is not more than ASCII code.
can any one help me to improve my knowledge.
For example as I passed "ABCD " it gets converted into 65,66,67,68.... I think this is not utf-8

UTF-8 is backwards compatible with ASCII of course. You should test with some characters that are not included in ASCII.
If you program in C# the strings are already encoded in UTF-16. You will not see anything Special there. If you want to see something you should try to compare the LENGTH of the Byte[] when you encode the string into different Encodings.

Check out the Wikipedia article on UTF8: Wikipedia.
From there:
Backward compatibility: One-byte codes are used only for the ASCII
values 0 through 127. In this case the UTF-8 code has the same value
as the ASCII code. The high-order bit of these codes is always 0. This
means that UTF-8 can be used for parsers expecting 8-bit extended
ASCII even if they are not designed for UTF-8.
The point here is that for anything that would be ASCII 0-127 in UTF8 it's the same. You need to try more extended characters (an example in the article is the Euro symbol) to see how it's different. Or try an ASCII value greater than 127 and you'll see it different.

Handle Non-UTF-8 Characters in Byte Array

I have an array of bytes which contains some characters that are not UTF-8. These characters cannot be deserialized using UTF-8 encoding. So, my question is, how can I handle these characters and make the string readable in whatever language it is.
For example, if I have an array:
byte[] b = myArrayWithNonUTF8Characters;
And I try to deserialize the array with:
DataContractJsonSerializer jsonSerializer = new DataContractJsonSerializer(typeof(MyObject));
MyObject objResponse = (MyObject)jsonSerializer.ReadObject(new MemoryStream(b));
Then I get an error that the array contains invalid UTF8 bytes.
Any way to make this work?
PS: Please, do not give me this answer: string s = System.Text.Encoding.UTF8.GetString(b, 0, b.Length); It will only return symbols replacing the non-UTF-8 characters.

The beauty of UTF is that it encodes characters in most languages; so you can have Greek and Japanese in the same character stream.
Without UTF, your entire stream (or in your case an array) must be in a single language defined by a Code Page. Each character is represented by an ASCII byte but the actual character is determined by the Code Page (see http://en.wikipedia.org/wiki/Code_page for more details).
For example if your text was written in Greek you might use Code Page 111:
System.Text.Encoding.GetEncoding(111)
In short, you need to know what language the ASCII text was written in.

ASCII Extended in C# string

How do I make a string in C# to accept non printable ASCII extended characters like • , cause when I try to put • in a string it just give a blank space or null.

Extended ASCII is just ASCII with the 8 high bits set to different values.
The problem lies in the fact that no commission has ratified a standard for extended ASCII. There are a lot of variants out there and there's no way to tell what you are using.
Now C# uses UTF-16 encoding which will be different from whichever extended ASCII you are using.
You will have to find the matching Unicode character and display it as follows
string a ="\u2649" ; //where 2649 is a the Unicode number
Console.write(a) ;
Alternatively you could find out which encoding your files use and use it like so
eg. encoding Windows-1252:
Encoding encoding = Encoding.GetEncoding(1252);
and for UTF-16
Encoding enc = new UnicodeEncoding(false, true, true);
and convert it using
Encoding.Convert (Encoding, Encoding, Byte[], Int32, Int32)
Details are here

Try this..
Convert those charcaters as string as folows.
string equivalentLetter = Encoding.Default.GetString(new byte[] { (byte)letter });
Now, the equivalent letter contains the correct string.
I tried this for EURO symbol, it worked.

.NET strings are UTF-16 encoded, not extended-ascii (whatever that is). By simply adding a number to a character will give you another defined character within the UTF-16 plain set. If you want to see the underlying character as it would be in your extended ASCII encoding you need to convert the newly calculated letter from whatever encoding you are talking about to UTF-16. See: http://msdn.microsoft.com/en-us/library/66sschk1.aspx

convert from ascii (dos) to windows

Hi I have a string written in ascii code whose output is " ”˜‰ƒ ‰™˜€" this is a name in Hebrew. How can I convert it to Hebrew letters?
.net c# winform

There are no Hebrew letters in ASCII, so you have to mean ANSI. There is a default encoding for the system that is used for encoding ANSI, which you need to know to decode it.
It's probably the Windows-1255 or ISO 8859-8 encoding that was used. You can use the Encoding class to decode the data. Example:
Encoding.GetEncoding("ISO 8859-8").GetString(data);
If you already have a string, the problem is that you have decoded data using the wrong encoding. You have to go further back in the process before the data was a string, so that you can get the actual encoded bytes.
If you for example are reading the string from a file, you have to either read the file as bytes instead, or set the encoding that the stream reader uses to decode the file data into characters.