That central dome is a dust cap. It keeps debris from entering the very small gap between the voice coil former and the center pole piece and the magnets. If you can stand the look of the dented piece (I know that is not easy to do), it is best to just leave it alone. Whatever means you do employ to pop it back into shape, take care not to exert force without also holding it firmly in place so that you don't tear it off. But, if you do tear it loose, you can probably glue it back without too much trouble, assuming you didn't tear up anything else in the process.