Battery replacement is standard on every vehicle on the planet.
1) Remove negative/ground (black) cable.
2) Remove positive (red) cable.
3) Pull out battery.
4) Put in new battery.
5) Replace positive cable.
6) Replace negative cable.
As a general warning though, if you have aftermarket CD players with anti-theft & a reset code, you'll have to put that code in before it will work. They are designed to shut down until you put the code in, every time it loses connection to the battery.
Also, if you have an onboard computer (like my 86 325e), you'll have to reset the units of measurement & time.