It's not necessary to remove the rim unless you are to do more than just replace the battery.
It would be good to start by saying, “Switch off the phone,” as most owners won't be accustomed to disassembling electronic devices.
It isn't necessary to remove the rim if all you are doing is changing the battery. That becomes necessary if you are to proceed farther into the phone.
It might be good to begin by saying switch the phone off. The Fairphone is for everybody and most will have no experience of disassembling electronic devices.