Disconnect the ribbon. There is a latch (possibly black - opposite from the ribbon side of the connector) you flip up on the main circuit board (blue arrow). This is easy to take apart but a pain to put back together. Make sure you have needle-nose pliers or tweezers for this when you put it back together.

The main circuit board is not the battery circuit board you first come to. As seen in the 2nd and 3rd images.

Blue arrow points to ribbon connector on main circuit board. White arrows on 2nd & 3rd images point to ribbon and connector latch on battery circuit board.

The connector can be damaged if you are not careful.