Use a suction handle to lift the back cover and create an opening pick to get in the gap.

Since the back cover is also slightly curved at the edges you want to insert the opening pick at a lower angle from underneath.

Start at the middle of the side and cut the adhesive while pulling up with the suction handle.

Be careful not to lever the opening pick too much to avoid breaking the back cover. Rather pull with the suction handle and use the pick for cutting the adhesive.

Move the pick towards the bottom corner and leave it there to keep the adhesive from resealing to the case.

Use another pick and move it to the top corner.

If it's getting harder to move the pick you might need to reheat the iOpener and apply it to the back cover again.