Hard to believe that there could be an acorn this early in the season large enough to damage anything. The Oak tree in our back yard doesn't drop acorns until sometime in the Fall.
To answer your real question, it is easy to replace the vent lid, but there is a need to get onto the roof.
1. From inside the bathroom, remove the plastic trim below the vent. On ours, one screw in each corner.
2. Remove vent lid crank handle and then remove the screen. On ours, two screws hold the screen in place.
3. Remove two screws from crank mechanism, which will allow the lift bar to be moved around so that the wheel at the end of the lift bar can be removed from the track on the inside of the lid.
4. Go up onto the roof and lift the lid back until it comes free of the hinge.
There are several hinge designs and it is best to take the old lid with you so you get a new lid with the right matching hinge.