Remove the battery.

To install a new battery:

Use a spudger or your fingers to peel any old adhesive off of the battery housing. The old adhesive shouldn't leave any residue.

If your new battery doesn't come with preinstalled adhesive, place a couple strips of double-sided tape into the battery housing.

Orient your new battery so its cables route through the cutout in the corner of the housing. Press the battery firmly into place to secure it.