Grab the bottom right edge of the leftover adhesive film and pull it towards the top of the phone with steady force to remove it.

To install a replacement battery:

Apply isopropyl alcohol (>90%) to the old adhesive and use the flat end of a spudger to scrape it off.

Remove the clear liner on the pre-cut battery adhesive and apply the sticky section to the battery well. Remove the opaque liner before continuing reassembly.

Remove the battery adhesive film from its liners and apply it to the battery.

Align the bottom edge of the battery (the edge without adhesive) with the bottom edge of the recess.

Lower the battery down and press firmly to secure it in place.