well my 20 Gb disk is now failing

Good thing I have a backup but still loosing a disc

I checked how much it costs to send this unit for Warrenty and its costs me more then just buy a new 20 gb disc, Thats of cource cause I live in Iceland,
Oh well I guess I can go to the store and buy new 40 Gb disc.

What is the best way for me to install a new disc ?