Required tools:

Replacement battery

T5 driver

Spudger or other plastic pry tool [You can use a flat bladed screwdriver if you're careful and don't mind some signs on the plastic finish]

Note: I did this documentation after having done the replacement, so you may notice that the battery pictured here is the original Garmin battery. Similarly, later in this documentation when we get the thing open, you'll see the third-party replacement already in place.