You need a burner.. just like a cd/dvd/blu-ray burner.. there is something called micro controller burner. you can attach that to your serial port.. then keep you ATMEL 89C51 micro controller into your burner.. you willl get a flash drive.. open it copy the code from the text file and save it into ur computer with extension .HEX..
then insert the new micro controller ATMEL 89C51.. then copy that into the new micro controller which was inserted.. in again .HEX format.. Thats it..
Note:
If you have an unlocked chip you can download the chip program and memory contents using the same programming software used to program it.
