Thanks a lot guys. DONE!!
For anyone searching who wants to know:
Connect keyboard and mouse and hit F1
Navigate to top level
To to 'etc' folder
Navigate to /userdata/systems/configs/emulationstation
Paste cfg file in this folder
Any changes you make in this will be reset once you restart computer, so make sure you copy the contents of the file, create a new file, paste it in, call the file something new then delete the cfg file you copied over and lastly rename this file back to es_systems.cfg
This breaks the link with the other cfg file in the etc folder, so it's not reset.
Then make your changes, go back to interface, refresh games list, and you're done. The changes will stick when you restart.