You have to be ‘devel-su’, this is like root on SFOS, then you can save edited system files. Don’t forget to make a backup of the original file every time you tweak system files.
To enable it, go to Settings - System - Developer tools - enable Developer Mode and Remote connection and give a password.
Have a look at the LAN IP address of the phone. Then you can SSH from Computer to this address in your LAN.
edit: If you’re in, type devel-su and enter the same password again. Then the terminal prompt changes from defaultuser to devel-su and gives you root rights.
See here for a long list of tweaks: