Although it's probably a good idea to remove the cover so you can have a look at the condition of the gears, you don't really have to if you just want to change the oil. Loosen up the bolts all around without removing them, and with a thin blade (putty knife) pry the cover away from the housing at the bottom, being very careful that you don't rip the gasket. The oil will drain out. Then you tighten the bolts again and fill'r up till the gear oil flows from the filler hole.
Bob S.