Too much water in box cake mix batter. Can it be fixed?
If you are only over by a small amount (¼ cup or less) just add half the volume of flour(±).  If you added a moderate amount of extra water(½ to 1 cup) add half the volume of flour and ½ tsp baking powder and sugar equal to ⅓ the flour. Cakes fixed this way will be slightly dry but serviceable, particularly given that you started with a boxed cake anyway.

Beyond that point just make a double recipe.