If you have proper reasons for invalidating a default judgment (incorrect service, wrong person, etc.) you can file a motion for voiding the judgment. To do this, you need to contact the court staff to determine the procedures.
However, you will need a valid reason for doing this.
As for appealing, there is normally a set amount of time (10 or 30 days) for appealing a judgment. Once that period has passed, the judgment cannot be appealed. If the default judgment was rendered recently, you can contact the court staff about filing an appeal.