It was set to manual. It didn’t download anything but only informs that an update is available.
To stop this, I found no option. That’s why I deleted the Jolla account on the phone. I assumed, that this way the phone can’t check for updates any more. But this didn’t help, update reminder is still there a few times a day.
Only guessing, I assume, maybe there is a boolian variable somewhere that says [available version] > [installed version] = true and therefore now shows the message eternally with or without Jolla account.
So my problem is how to set it to false and stop the message, if it’s really this way.