Using two hands, place fingers under plastic [6" long cover] near base and pull apart [1/8"] slightly and slide away from base. There are moulded edges about 1/2" long inside the cover. If you break it, it's about $10 from the dealer, cheeper from the junk yard.

If you're talking about the round cover, you have to take out the entire wiper assembly, that you have to do anyway to clean & lube the mechanism.