If you're talking about a vent cover that allows the roof vent to be raised even in the rain, there are different types. At worst you might have to find screws under caulk and remove.
If you'r taling about the square domed cover that seals the 14" square, they usually have screws into the hinge. Crank it up all the way and either attack from below if you can remove the screen or from above. Replacement covers aren't too expensive, take the old one with so you can get a better match.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI