I’m having exactly the same issue. I’m trying to upgrade a Connect to S2 and it has been stuck on about 75% of the way around the progress circle for about an hour. I am reluctant to power down and try to start again because I’ve got no idea what state the device is in right now if it had to restart.

Did you ever find a solution?

Or can anyone else help, please?

In the grand scheme of things (and there is a list of slower devices to update, likely due to the older, slower CPU in them), if it has been a couple of hours, I would be tempted to do a full factory reset , although you may want to call Sonos Support directly to discuss it before taking that step.