How do I upgrade CPU?

So I'm looking at buying this computer from someone for cheap and most of the components look pretty good except the cpu is kinda slow. Its a Core2Duo 6400 2.13GHZ. He has a Gigabyte GA965P motherboard in his system and I want to upgrade the cpu to an 3 ghz intel core2duo or maybe a core2quad.

So I need to know how exactly I go about doing this (how do I replace the old cpu with a new one), and what cpu do people recommend for best price/performance ratio?

forgot to mention; this is a computer that i am buying for a friend (she doesn't know anything about computers so i am trying to get her a good deal on a decent computer). She mostly uses itunes, the web, some word processing, and your other everyday stuff. she isn't a big gamer. she wants a computer that will be good enough to run apps well for several years to come, preferably something that can multitask fairly well. (last time she bought a computer was like 6 years ago).

if that's all she uses her computer for, and she can keep it for that long, then an E6400 would be plenty.

it depends on the rev of the MB GA-965P-DS3, rev 1.0 & 2.0 don't support the q9550.
Rev 1.0 is limited to 65mn CPU with a max of 1066 fsb.
Rev 2.0 support some 45mn CPU with a 1333 OC fsb.
Rev 3.0 support almost all but non officially the 45mn at max 1333 fsb.