To remove the doorbell from its back plate, use the Opening Key that came taped to the paper template in the product box. If you need another key, contact Blink support.
Put the key into the opening at the base of the Blink Video Doorbell.
Lift, or pull the key 'toward' you while you move the doorbell up (toward the top, or camera side of the doorbell) to unseat the hooks and separate from the back plate.
Inside the battery case you can find the QR code and corresponding Device Serial Number (DSN), the reset button, and a diagram shows the correct position to insert new batteries.
Reconnect the doorbell to the backplate by lining up the pins and using a two step motion - press the doorbell against the backplate firmly (with moderate force), then push down to line up the bottom edges. You should feel a slight click when the doorbell is secure.
Press the doorbell button to test it.