Horrible job, so arm yourself with cloths, drip tray and some disposable gloves cos it's going to go everywhere.
Take the two screws out and drain the oil. Clean the magnet on the inside of the screws of any swarf. Dealers will use a pump but at home you will need probably 3 tubes of gear oil. Cut the tops off each tube ready for action. Squirt the first tube into the lower
hole and then without it running out again
quickly put the next tube in and continue filling the box up from the lower
drain hole. When oil runs out of the top hole its full and more importantly you have avoided trapping air pockets inside.