How do I swap my Ford Ka casette player stereo with a Ford Ka Cd player st?

I recently bought a 2001 Ford Ka.
The stereo is locked with a code which we don't know, plus it only has a casette player.
I'd like to swap this stereo with another Ford stereo that has a CD player instead.
Does anybody know how I go about doing this, as I honestly have no idea really!
Thanks.