In many cases you can just unscrew the flange from the floor, lift it, shim under it and re-screw. This is most likely possible if it is modern, i.e. pvc pipes and you have access below to tighten hanging support instead of letting it all hang from flange. 6 of one 1/2 of another. The special "extension flanges" work fine. If it is just a little bit too low I have seen people cheat by using two wax rings - probably the lease attractive option.