When it was taking too long on the Mini (another update), I tried to turn WIFI off and then logged out of my account. When logging in again, the update started again and went through fine.
If I couldn't turn WIFI off, I would have tried to turn the Mini off by holding the power slider for more than 10 seconds.
The main thing was always to log out/log in to clear the database.
Disclaimer: I don't know if by doing that way one can brick the device, it always worked for me.
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