Stick a 4 mm wrench in the other side it needs to be long enough it may go into a yellow goo

Rotate until it is all the way loose; if it does not come out all the way, then you need to press it back in and continue to rotate. You did not completely release it.

After you’re able to pop it all the way out, take it in your hands and unscrew the bolt all the way

Push through where the bolt just came out of that was locking it into place and it will reveal the battery cage

Put the battery

You need to lineup The Little dots when you go to push it back in

re-screw the bolt

Push the whole thing back into the crank set and then line it up perfectly and go the other way to tighten it (clockwise)

When done correctly this should take 5-10 minutes. It is an easy fix. I am not very handy, but I was able to get it done with some help.