If only the area directly below the flange is rotted you can install a two piece steel closet flange support.
Installing toilet flange on wood floor.
But if your floor is severely rotted say more than an inch beyond the flange you re stuck replacing the flooring around the toilet.
In this video i demonstrate a simple and permanent method of installing a toilet where the new tile floor was installed over the original subfloor but the to.
Rest the toilet bowl on its side place it on a towel or cloth to protect the floor and attach the replacement wax ring to the bottom of the bowl this technique helps ensure the ring is in the proper position when placing the toilet over the flange.
There are two options this step.