procedure TForm1.Button1Click(Sender: TObject);
Var F       :File;
       Buff :pointer;
       arr   :array of char;
       size,fRead,fWrite,i:Integer;
begin

   AssignFile(F,'C:\WORK\Data.txt');
   reset(F,1);
   size:=FileSize(F); 

   GetMem(Buff,size);

   BlockRead(F,Buff^,size,fRead);
   arr:=Buff;
   for i:=0 to fRead-1 do
        if arr[i] in ['a','b','c'] then arr[i]:='X';

   CloseFile(F);

   AssignFile(F,'C:\WORK\Data.txt');
   rewrite(F,1);
   BlockWrite(F,Buff^,size,fWrite);
  
   CloseFile(F);

   FreeMem(Buff);

end;
