This granny is happy to give you advise.
How this is attached and what kind of material your drain pipes are made of determine how it is repaired. How have you determined that you need a new closet flange? You may be better off checking out flange repair kits that are available.
To replace the flange, all the concrete around the flange will need to be chipped away (this requires at the very least a hammer drill). You need to remove enough concrete so that you have room to work. You need to be able to cut the old flange out and reattach--you may have to go to a horizontal pipe since alot of closet flanges are close connected to the closet bend. Each different type of material has its own special glue. The closet flange sits on top of the finish floor. The closet flange MUST be bolted to the floor with concrete anchors. The toilet bolts to that.
So, if you cannot get a repair flange ring to work, this may be beyond something that you want to tackle. Replacing a flange on concrete is NOT fun.