Hold the back glass upright with one hand and rotate it just enough to reveal the battery press connector just below the volume buttons.

Use the tip of an opening pick to pry up and disconnect the battery press connector from the logic board.

A plus and minus sign is printed on the head of this connector.

To reconnect a press connector, align it over its socket and gently press down on one side until it clicks into place, then press down on the other side. Make sure it sits flat, as the insulating rubber around the sockets might prevent it from connecting fully.