Once the faucet body is removed, you can then access the locking nut that is probably holding the cartridge down. Some models of faucets don't have a lock nut but most do. Loosen and remove the nut by using a pipe wrench as shown here or Channel-Lock type pliers.
Note the brass screw in the top of this cartridge. Its purpose is to reinforce the stem when the faucet body's allen screw is tightened against it. You may need to remove this old screw and reuse it in the new cartridge as not all replacement cartridges come with a new screw.
