Changing your differential fluid is as easy as changing oil.
Put a pan under the axle and remove all the bolts in the cover, except the top one. Loosen the top bolt but leave it in. Try pulling up on the cover gently, if it won't come loose use a small flat head screw driver, and use a hammer to tap it inbetween the cover and the axle.
Once all the fluid has drained go ahead and take out the top bolt and remove the cover. Using a scrapper, clean any old silicone or gasket from the cover and axle housing.
Once you have the cover and the housing clean, spray them with brake cleaner and wipe them down with a clean rag. Once you think you have them clean, go ahead and wipe them down one more time.
Once you have both the cover and the housing clean lay a continous bead of silicone on the inside of the bolt holes.
Then using your finger (your hands should be clean) smear the silicone over the cover so it covers the whole mounting flange. If you need to add silicone in places thats fine.
Then place two bolts in the cover, doesn't matter where, I use the top and bottom. Set the cover in place and start these bolts. Using the two bolts will line the cover up where it should be so you don't have to hunt for the holes.
Start putting the bolts back in. These bolts don't need to be super tight, just snug, so use a small ratchet. Work your way around the cover, once you have them all in, go back around them one more time to make sure they are all evenly tightened.
Give the silicone a few minutes to setup then refill the diff. Always just fill them till oil comes out the fill hole. If there isn't a fill hole on the cover, there will be one on the front side of the axle.