Rubber ring. What brand? I think the easiest way is to shut off water, disconnect the hose to the bowl, remove the 2 screws that hold the bowl. Once the bowl is removed it is real easy to lift and maneuver the bottom pedestal and install new mounting.
Go to the manufacturer web site and locate the proper parts first. I had a difficult time finding an RV store that had the parts I needed. Locating the parts were more difficult than the repair. Once you have it apart you may just want to replace the bowl seal also. It was easier for me to locate the complete gasket kit.