unscrew the fill bolt (1st!) then the drain bolt on the differental. Once its all drained out, screw in the drain bolt and fill with your favorite brand of gear oil (75w90 or 80w90) until its full (ie. fluid coming out of the fill hole). Screw the fill screw back in. Wash floor and hands with degreaser.

The fill/drain screws are a 14mm hex key (I think...thats what they are on the 124 anway)