software/firmware woes, MKS Gen 2z board, Prusa i3

So, I've been printing tons of trinkets and such for the past while, and haven't really bothered to do any serious stuff. Unfortunately, my next project requires precision, so I must finally dig into the fine tuning. My latest 20mm calibration cube measured x=19.80, y=19.80, z=20.55....

I am running a cheap Prusa i3 kit (purchased roughly 2015? Upgraded e3D hot end) that came with the MKS Gen 2z board, and Repetier 1.0.6. It works, and I don't want to mess with any of that. Strangely enough, however, I can't seem to access the EEPROM settings through Repetier, and can't figure out any other way to get into the steps settings to tune my x, y, and z. All of the Google resources tell me I have to flash new firmware to the board using Merlin? Is there any way of copying the current firmware in case this doesn't work or drastically screws something up? It currently works for most things, and I don't really want to mess with that too much... I just need to tune the steps a little! :(

Thanks!
- CK