If you’ve moved to iOS 12.1, it’s no longer possible to go back to any previous version. Not that there’s much reason to.
As of today, you can’t install iOS 12.0.1 or earlier on an iPhone or iPad. This in-line with Apple’s standard practices.
iOS 12.1 debuted last month, bringing Group FaceTime and 70 new emoji. It replaced iOS 12.0.1, a tweaked version of iOS 12, which came out in September.
Version 12.1 has been out long enough for any significant problems to have been discovered. As none turned up, Apple no longer sees a reason for people to downgrade to iOS 12.0.1. So it’s no longer possible.
A beginner’s guide to downgrading
It’s self evident that moving between iOS versions — up or down — requires a copy of the replacement operating system. Obviously, the only source for these is Apple.
Each new copy must be “signed” for the iPhone or iPad to trust it. As of today, Apple stopped signing copies of iOS 12.0.1.
iOS 12.1 has proved to be quite stable in the four weeks since it came out. Nevertheless, anyone who’s just now installing it should be aware that it’s not possible to go back.
The release of the next version, iOS 12.1.1, isn’t that far away — Apple has been beta testing it for weeks. This is just includes tweaks and bug fixes, not new features.