Hardest part is getting the split pin out if taking them off, its a bit of a tight squeeze. They are the same in principal as WARN hubs.
Undo the six screws round the edge, and ease the spring out, and the cap off. It all comes out as one, and there is a paper gasket between the edge with the screws and the main body. (cartridge paper, makes good gaskets, or cornflake packets) that gives you access to the castle nut. Remove the split pin, then undo the castle nut and remove. Dont forget the felt washer, you'll need a new one. Then undo the six bolts holding the FWH to the hub, again there is a paper gasket here, same as any other hub, standard or FWH.
There is an O ring on the inner edge of the hub body, you'll need to replace that. Any decent garage should have one, though an HGV garage may be a better bet.
Thats about it....