Nick doesn't want to update to the Windows 10 1803 update. How can he keep from updating? Leo says that he can defer it by telling Windows he's on a metered connection (in network settings), but ultimately, he's going to have to bite the bullet. Leo says that he can save the update on a thumb drive, but Avast could be causing issues with the update. Nick should check out this article on windowscentral.com for common problems and fixes.
There is also a bug where 1803 is bricking the computer. It requires entering advanced startup and then try rolling back to the previous build. Here's a list of the bugs at windowslatest.com.
Otherwise, his only real choice is to backup his data and then reformat the drive and reinstall Windows. Leo also says he should stop using Avast! He doesn't need it.