Underneath your toilet is a wax ring that compresses to keep the waste from leaking out. The wax compresses against the underside of the toilet and the flange that the toilet bolts to. I get homeowners that ask “how often should I replace the wax ring?”
It’s not a difficult job, but if you’ve never done it before, you might want to get some light instruction first. My answer is you shouldn’t replace the wax ring unless you are removing or replacing the toilet or if you have a leak or suspect a leak from the toilet area.
The wax shouldn’t degrade and should therefore keep a tight seal. The thing to watch out for is movement where the bowl meets the floor. If you get any movement of the bowl, the seal is breached and you can get leaking when you flush the toilet. Sometimes the toilet bolts will corrode and lose their ability to hold the toilet to the floor and the toilet can move. Also if the toilet holes weren’t directly over the flange holes, the bolts may be at a slight angle, which can allow some movement. Unless you see a leak issue at the toilet, I wouldn’t replace the wax ring.