How do I remove the factory stereo? on 1991 BMW 325i

A battery cable came undone and now I need to enter a code to make the stereo work again. I know I need the serial number on the back of the stereo in order to retrieve the code. How do I remove the stereo?

Asked by for the 1991 BMW 325i
Normally in most cases those are in owners manuals - codes. Was on mine.
I have the owner's manual. However, the manual states there is another manual just for the stereo and I don't have that one. Any other suggestions?