Hi @oukkola, welcome to the forum 
Our old Companion software is no longer being maintained or supported, so we would recommend installing BlueOS instead.
Assuming your onboard computer is connected to the internet, then the issue may be because you’re using a Companion version that’s too old to securely communicate with the ArduPilot firmware server. Companion 0.0.30 added a workaround for this which just doesn’t attempt a secure connection.
If you don’t want to install BlueOS then you can try updating your Companion software before you try to update the autopilot firmware, or you can update the autopilot by directly connecting the flight controller to a computer running QGroundControl.