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.

During reassembly, follow these instructions to apply a new battery adhesive film:

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.