Hi,
I have atatched a couple of photos from the internet showing where the battery is located for a hybrid- you have to remove the rear curved shape cover which is accessed from behind the front seats- that is from the side door- it is very awkward and you have to push the centre seats well back. The cover clips off so use a special trim removing tool if you have one but it may come off by pulling on the "side bumps" on the cover- it's also a devil to get back on. You have to remove the vertical bar first and wiggle the battery out a bit to get to the terminal nuts- the battery is a very snug fit so make sure you have the right battery and must be a stop start type- an ordinary battery will likely fail after a while which is what happened to me- I did put a post on a while ago which gave the battery I had to get (for a 2005 E4). Don't forget to reset the door window switches - see Youtube - good luck