Forcing StreamWriter to change Encoding

Just wrap it in a FileStream.

StreamWriter sw = new StreamWriter(
    new FileStream(saveFileDialog1.FileName, FileMode.Open, FileAccess.ReadWrite),
    Encoding.UTF8
);

If you want to append, use FileMode.Append instead.

You should also call Dispose() on a try/finally block, or use a using block to dispose the object when it exceeds the using scope:

using(
    var sw = new StreamWriter(
        new FileStream(saveFileDialog1.FileName, FileMode.Open, FileAccess.ReadWrite),
        Encoding.UTF8
    )
)
{
    sw.Write(sb.ToString());
}

This will properly close and dispose the streams across all exception paths.

UPDATE:

As per JinThakur's comment below, there is a constructor overload for StreamWriter that lets you do this directly:

var sw = new StreamWriter(saveFileDialog1.FileName, false, Encoding.UTF8);

The second parameter specifies whether the StreamWriter should append to the file if it exists, rather than truncating it.

setting UTF8 encoding working with Arabic font is the best thing I did:

 using (var sw = new StreamWriter(

 new FileStream(temporaryFilePath,    
               FileMode.Create,
               FileAccess.ReadWrite), 
               Encoding.UTF8))
            {
                sw.Write(sb.ToString());
            }
 )

There is a constructor for filename, appendMode, encoding.

With a proper using block it looks like:

if (dr == DialogResult.OK)
{
    using (StreamWriter sw = new StreamWriter(saveFileDialog1.FileName, 
           false, Encoding.UTF8))
    {
      sw.Write(sb.ToString());
      //sw.Close();
    }
}

There is a StreamWriter(string path, bool append, Encoding encoding) constructor - you could just explicitly specify the append flag too?

I said you ought to wrap your StreamWriter in a using too, i.e.

if (dr == DialogResult.OK)
{
    using(StreamWriter sw = new StreamWriter(saveFileDialog1.FileName, false, Encoding.UTF8)) {
        sw.Write(sb.ToString());
        sw.Close();
    }
}

although realistically this won't make any difference here. This effectively puts a try/finally around the code so that the StreamWriter will get cleaned up (it'll call sw.Dispose() even if an exception gets thrown in the meantime. (Some people will say this also means you no longer need the .Close since the Dispose will take care of that too but I prefer to have it anyway.)