screws driving me mad

[Dickylew]

Hi im trying to take out the center diff in my Hobao Hyper st black so that i can replace the factory grease with proper diff oil.

Ive tried to take the four screws out of the bottom that hold it in place but they just keep going around and ondoing the top screws instead, im guessing that the thread the screws go into is free to rotate.

Can anyone tell me how to get these screws out please.