Just open them up. Most important thing is to remember how they go back together. lol

It's pretty easy for anyone that has a knack for things mechanical. I'm a computer tech by trade so I rebuild laptops and destop computers on a regular bassis. I just use the same mentality for this. Take it apart, lay the parts out in order as you remove them and then reassemble, but apply grease. I use dielectric grease myself. It's synthetic, has no ill effect on the plastics and last a long time. It's a bit thick and causes the diffs to not slip as much as oil, but Idon't race so it's not a big deal for my diffs to grab.

You pretty much just start at the diff housing and remove screws. The diffs themselves will have small gears in them. Similar to this.

Just either fill this section with diff oil or squirt some grease in there between the gears and reassemble. You can pull all of the gears out and clean the old lubricant off if you like. I usually do.