Use an opening pick to cut the adhesive on all sides of the phone. Leave one opening pick in every corner to prevent the adhesive from resealing.

Reuse an iOpener if the adhesive becomes hard to cut. During the removal process the back cover is under tension all the time and is likely to break if the adhesive isn't loose enough.

Insert an opening pick on the right side of the phone after you cut the adhesive on all sides. Flip it sideways until you can get a good grip on the back cover.