how do i change the battery?

Not sure if it's the same as my 02 325, I am assuming it's probably similar. The battery is in the trunk, in the right rear. Peel back the panel that covers it, there are two black screws that hold the battery in place. Remove those. You will need to remove both battery cables with a 10mm wrench. There is a bolt that holds the battery in place underneath the actual battery so you will need to remove that too. Once that bolt is removed you should be able to pull the battery out and do the whole process in reverse.