How do I replace the main circuit board?

I've replaces mot of the parts in the Roomba 690 over time, including the brush motor, so I've figured out how to disassemble most of it. However this time it looks like the main board is faulty, as the the Roomba refuses to recharge. I sometimes get a charging error 3 or 6 but usually it just chirps every couple of minutes.

I had been able to connect occasionally through WiFi, which showed that the battery was too low to start a job. However, after trying a reset, I can't get it to reconnect.

I've figured out how to remove most of the parts, so there are no screws visible from below and I've got everything off the top that I can, including the top arc piece that seems to be decorative - it's got hinges at each end but no electrical connection. But I can get the plastic circle off the top (the part that surrounds the buttons) nor the main side panel (the part that goes from one side of the brush bay right the way around to the other side and also holds the bumper in place). I can pry it off a little all the away along its outside edge, but something it still holding it in place.

I suspect that there could be some screws securing it that are covered by the plastic disc surrounding the control buttons, but that piece seems to be held even more securely.

Any ideas? The main board is under there but I can't figure out how to get to it.