ChangeFileExt changes the extension of a file name.

Example:
When you type the new extension into Edit1, put the "." in first. An example of this is   .txt

procedure TForm1.Button5Click(Sender: TObject);
var S1 : String;
begin
  if OpenDialog1.Execute then
   Memo1.Lines.LoadFromFile(OpenDialog1.FileName);
   S1 := ChangeFileExt(OpenDialog1.FileName, Edit1.Text);
   Memo1.Lines.SaveToFile(S1);
end;