The two batteries are soldered on the board. I hope when new batteries are soldered on the board in the future, it will resume working normally.
So there is no battery change without tools.
EDIT: It's ER14505 form factor Li-SOCI2. But the welded solder pads seem to be unusual, 2x through hole on the negative, 1x through hole on the plus.
Those living far away from support will have to replace them in-house when the time comes.