You can use the Seek method of the BaseStream property of StreamWriter class to jump to the location.
sample code
FileStream stream = new FileStream(@"filename.txt",FileMode.Open,FileAccess.ReadWrite);
StreamWriter writer = new StreamWriter(stream);
writer.BaseStream.Seek(10000, SeekOrigin.Begin);
writer.WriteLine("new Test starts here");
Check
MSDN-Seek[
^] and
BaseStream[
^] links for more details