Автор: http://www.swissdelphicenter.ch

// Replaces a string in a file with new string. 
// Ersetzt eine Zeichenkette in einer Datei mit einer anderen Zeichenkette. 

procedure TForm1.Button1Click(Sender: TObject);
 var
   f: file;
   l: Longint;
   FileName, oldstring, newstring, s: string;
 begin
   oldstring := 'old string';
   newstring := 'new string';
   FileName  := 'c:\YourFileName.xyz';

      s := oldstring;
   AssignFile(f, FileName);
   Reset(f, 1);
   for l := 0 to FileSize(f) - Length(oldstring) - 1 do
   begin
     Application.ProcessMessages;
     Seek(f, l);
     BlockRead(f, oldstring[1], Length(oldstring));
     if oldstring = s then
     begin
       Seek(f, l);
       BlockWrite(f, newstring[1], Length(newstring));
       ShowMessage('String successfully replaced!');
     end;
     Application.ProcessMessages;
   end;
   CloseFile(f);
 end;