@Philbert thank you! I though that connecting through tplinkcloud I could also see the manual upgrade option, but it is not the case (at least by default). I have therefore just logged directly on the controller using the local IP. Kudos 

  @giopas 

NEVER perform any upgrades when being connected via TP-Link Cloud.

ALWAYS connect to local IP's and then do the desired upgrade.

There have been many reports in the past that devices might become unresponsive/stuck in upgrade process/bricked when starting upgrade through TP Link Cloud.

Just as a hint.

 Thank you both!

Question: how can I go from the beta to the new stable? Will that upgrade work?

  @giopas 

Upgrades will work.

First, log into your controller (local IP!) and go to devices page, from there you should be able to upgrade your router to latest stable version.

Once this is done, the router rebooted and is visible again on your devices page showing latest firmware, only then go ahead and upgrade your OC200 via Controller Maintenance page in the Menu.

Should be pretty much straight forward.

Nobody asked me, but I had a similar problem: could not find the upgrade button. I tried some wrong stuff (like creating a local account), before understanding I needed to log on using the IP from the local network (logging from the cloud I couldn't even see the option for manual upgrade), and using the cloud account (instead of the local one I created); only then I was able to see the upgrade button.

Then I checked for upgrades, and it still came back empty. So what I did was:

1) backup of the settings (I could not download it, so I had to set up an SSH server)

2) upgrade using the .bin file (the one I got from Download for OC200 | TP-Link)

3) log back in using the cloud master account, and then restore the settings

And it worked.

I have the controller running on hardware (OC200 v2), so I checked that I had the right file a couple of times before starting everything. But it worked out in the end.