Switch off the doorbell installed on your door. Unscrew the face plate and turn it around and disconnect all the connecting wires. Detach the doorbell button as well.
Attach the mounting bracket on the wall over the wires of the doorbell.
Take the disconnected wires of the doorbell and connect them to the AC adapter of the buzzer. Plug the adapter into an electrical outlet nearby.
Disconnect any other wires that are connected to the doorbell. Secure the disconnected wires by wrapping electrical tape around the ends of them.
Install the door buzzer on the wall and place it on the mounting bracket so that it will be supported by the bracket. Screw the buzzer in place. Plug the intercom in and turn it on.