Sometimes the push buttons will get stuck when pushed, from dirt etc around the holes. Try cleaning with some compressed air. If it is stuck on, it would probably interfere with the upgrade.
Pull the battery out of the controller and put it back in. Make sure the drone and controller are paired. If not, the controller will say Waiting for Solo. If you're trying to update the firmware, download the SidePilot manual and follow the instructions for installing Open Solo 4 in the Firmware section. It is important to follow the steps and take your time.