Replacing Tub And Shower faucet cartridges is quite easy. Most cartridges are held in place with a threaded nut. Loosen the nut and out comes the cartridge. The retaining nut is usually behind the shower handle which comes of by unscrewing the hex that holds the handle onto the cartridge.
See the detailed video below on how to remove the handle, trim plate, and cartridge. Also, make sure that when reinserting the cartridge the rubber seal is not damaged.