Just use it to reduce the diameter of the ring. Pull the ring, and then you can gently remove the battery.
Then disconnect the battery, reconnect the new one, replace the ring and re-screw the bottom. Done!
I've done that with 2 Beocom 2 MK1, but there was 2 types of "internal ring" (I've got pictures, but I'm not at home) one was a disc, the other was a ring. If it's the disc, you just have to turn to release. But it's a bit tricky to replace everything so make sure you remember the EXACT position of the disc. Then you will have to re-screw the bottom on the disc, that's the tricky part. If something is misplaced, it doesn't work...
Has anyone found a source for "normally priced" Beo5 batteries?