Clover and I replaced the battery in @susan’s keyfob! We used the instructions at Toyota Prius: How to Change the Key FOB Battery - PriusDIY.com, and were able to deconstruct the thing and replace the battery, a standard CR2032 flat round battery.
Finding information about the 2006 Prius is slow going, but when I do find info it is usually very good.
And Clover seriously helped, used 000 screwdriver head to get the tiny screws out and back in and everything. It was great! Because this is like, the fifth category of items we’ve deconstructed. “Laptops” were not in general circulation when I was 9, and yet Clover has seen multiple laptops opened up and repaired, at home. I’m certainly hitting my childhood goals!