You can cut it with scissors. You can cut it with a knife. You can use bone cutters or something similar and cut off a tiny piece of rock with it. You can take the rock out of the water and saw it off, or chisel it off.
Ideally you want to get some rock with it as it is a pain to attach but it can be done loose. If you get it lose just keep it under very low flow with some rubble to attach. Usually I put a water bottle or something similar in the tank with rubble and loose pieces and after a few days they will attach. You can use glue but the glue is just helping it won't hold it together on it's own.
Lots and lots and lots of ways to go about this. If you are nervous just start with a small piece. Keep in mind it's very slimy.
It's incredibly hardy (also slimy) so really any way you decide to cut it should end up just fine. Any pieces left on the rock (or any that end up settling in your system) will likely just grow into new polyps. I'd definitely recommend fragmenting it in a separate container. But either way it's easy to cut.