Using an iOpener or heat gun, warm the adhesive around the edges of the back cover to soften it.

Once softened, run an opening pick or guitar pick around the edge of the phone to separate the back cover from the phone.

If you are unable to wedge the pick underneath the cover, you could try to shave down the pick to make it thinner or use a sharp instrument to get it started. Once you pry it open enough to get a pick underneath, use that exclusively.

If you need to use a sharp instrument, there is much less chance of damaging delicate internals by starting at the bottom of the phone because the speaker array is fully encased in plastic.