Just follow the CakePHP Migration guide and update to 2.3.
Honestly, I didn't even read through the migration guide, I just swapped into 2.3 and everything has just worked. It doesn't appear that there are many changes to existing code - just improvements / additions, so you'll likely not have to do any code modification.
I like to keep my versions of Cake separate (see advanced installation), but if you're on the normal install, just replace the files within /lib/Cake/
with the new versions files.