Not able to upgrade to iOS 12 from iOS12 Beta
Solution 1:
This is an Apple bug that looks like it is fairly wide spread.
Apparent iOS 12 beta bug causes never-ending notifications to update software
Solution 2:
Apple has provided a fix as of today. (2018-8-31)
iOS 12 public beta 10 (build 16A5366a or newer should do the trick)
https://apple.news/AwGLjBb1sSwivkwnb3d7-VA
Solution 3:
Change the date back a day or three (or more as time marches in) and that will temporarily prevent the dialog.