How can a overfilled cavity filling be fixed?

See your denist: They should be able to sand down the filling to get your bite to set properly.

By being replaced: If by overfilled you mean that the filling is too big and you cannot chew, the filling can be shaved down. If overfilled means you think it is too deep rendering the tooth too sensitive the only solution is removal of the nerve of the tooth, usually done with root canal therapy.