Linux

Your power button is the chunky button that powers the pc on and off. Typically you'd push and hold it for several seconds until your computer shuts off, ditto for starting again.

It probably looks like one of these and is on the front or top of your computer case near the front, odds aren't has an LED.

It's usually advised not to press it unless you have to, since apps like to close and save and stuff before they get shut down so the button doesn't give your pc time or notice to do that.

Luckily in the "start menu" or whatever UI element where there are buttons for navigating the list of apps there is usually a built in button for shutting down the pc, saving the currently opened stuff to RAM and putting the pc into "sleep", possibly a similar function called "hibernate" too. On Pop if you are using GNOME or (I think) Cosmic as your desktop, those elements aren't the top bar in the upper right. Otherwise you should be able to simply search "shut down" in whatever search menu/launcher pops up if you hit the Alt key.

Your firmware update might ask you specifically to use the power button on the outside of your pc case, but it should prompt you on screen if that's necessary.

I hope this helps! You probably wanna get any updates you've been putting off done, it could even help without mouse issue.