Filling and recreating toilet bolts in concrete

You should be able to remove the ring and replace it. This is true for most steel rings but not necessarily for some plastic or brass rings. It is possible to add a stainless steel replacement ring over the top of the old ring if it's not already more than 1/4" proud of the floor. You should be able to find replacement rings and flange repair kits at your local hardware or builders supply store that will work for your specific situation.