Peel off the two black stickers on the right side of the battery using your fingers.

Remove the battery by pulling the right side of it upwards.

The black stickers are the protruding ends of the two parallel adhesive strips which hold the battery in place. You can see the adhesive strips in the third image. Focus your effort on these regions.

If you did not remove the SIM/SD card reader in the previous step, you will need to carefully slide the battery ribbon cable out from under it once you have freed the battery from the adhesive strips.