Unless you have some type of aftermarket diff cover. The plug that you are looking at is more than likely going to be a fill plug and not a drain plug.
You will need to take the bolts off of the cover and take the cover off to drain your fluid.
Then after you have drained the fluid. You'll need to scrape off both the cover and the housing and get rid of all the old RTV.
Then you will need to apply new RTV. I usually put it on the cover and then I put the cover on the housing and put the bolts in and tighten everything down.
Then add the new fluid into the fill plug.
If you actually have a aftermarket cover that has a drain plug then you will not need RTV just drain and refill.
Friction modifier is needed if you have a limited slip.
mac 'diffs r us' gyvr