Most likely the bullet slides out of the ring, and you can change the batteries in it. I will try to find out for sure, everyone has left EF central for the day.
I own a version of this ring (I don't think it's the number 7. It's another version) and the bullet doesn't really seem to want to come out. It's looking like it's probably only going to last as long as the battery does.
The manufacturer does suggest not to attempt and remove the batteries. But they do say that the device requires 2 × 3G-A CNB batteries. I believe technically it would be possible, although not recommended.
jdl_131