I have the same issue with beta 3. I was never able to get past it so I waited for Beta 4. Beta 3 stuck at 15 minutes, beta 4 sticks at 10 minutes. It has been on 10 minutes since yesterday afternoon. I would like to remove the update file that is downloaded and then do the update again but I do not know where it is on my Mac.

Thank you Neto. Unfortunately, the reboot did not work. I tried multiple times.