Good news for you is the OP6 isn't water resistant so doing it yourself will not destroy ay key feature!
The back glass is not hard to get off but if you you fear breaking it you can get one on Amazon for just over $15 and is has new adhesive so it will secury stick. The only thing you need to ensure is you get an real battery and not some cheap knock off battery!
I have butter hands and have dropped my phone twice. THIS cover is an exact replacement. And it's super easy to replace. If you are at all considering getting a battery now would be the time to change it if you're taking your back cover off the battery is right there. I also got my battery from ifixit.com which is also a direct replacement and it works just as good as the factory battery.
Super easy fix, I've replaced mine 3 times now.
Edit: Here's the last one I ordered. Comes with all you need to remove the old one. All you need is a hair dryer to heat up the glue to pull it off.
https://www.amazon.com/dp/B08285SJMJ?ref=ppx_pop_mob_ap_share
Ifixit battery is legit and it is very easy to change. If not careful the back glass panel may crack when removing it so I suggest just buying a new panel and it will come with all the tools needed to take it off which means you have all the tools to change the battery aside from the very small philps/slotted screw driver for the battery plug.
Seriously easy job to do.