ROM hacking can be complicated or simple. Some people have created "patches" for ROMs. What these are is
files used to hack into a game and change something. Patches have been made for translations, changing levels, and a
bunch of other random stuff. To "patch" a ROM you need a patching tool. They're simple to use. Enter the
patch file, enter the ROM file and get a hacked ROM.
You can also do your own hacking. Zophar has plenty of utilities for doing your own hacking. Most of them
can be used to change levels, sprites (the game characters), text and other stuff. If you really want to get creative
get hex editors and corruptors. These can be used to change just about everything. I never use them but if you
want to you can. And please don't aske me how to use them, I dont' know.