This is a type of fluid that's rarely touched by others. However, Differential Fluid is as important as other fluid that runs for your car such as engine and transmission fluid.
So What's a differential and why do I need it? Basically it allows your car wheels to spin at different speed as your go around the corner. If your car has a differential installed, the inside wheel will spin slower than the outside wheel during cornering. The gears inside your differential help your engine to distribute the energy to the axles and then to your wheels. This is why it's called differential. Just like the engine oil, your differential oil must also be changed regularly but not as often as the engine oil.
There are some consideration before you change your differential fluid. Differential fluid comes with different weight, or viscosity, and GL rating. Before you purchase your differential fluid, you would need to find out the weight and GL rating for proper lubrication to your differential gears. Check your local auto store or your service manual for the correct specified differential fluid. Once you have bought the correct fluid, then changing the differential fluid is very similar to changing your engine oil.
See the video below for better instruction:
How to Drain Differential Oil for Rear Axle
What Type of Differential Fluid to Use