Let's say you have a cheat entered into the list where you can toggle it on/off (Y/N). If you highlight that cheat and press square, it will go to another screen where you can modify the value. If you press triangle at that point, it will take you directly to that address in memory so you can manually change it in running memory.
If you do modify the cheat itself, remember to press select when you're done if you want to save the database. And if you ever replace the database, it will reset.