How to downgrade from any version to iOS 11.3.1 without losing data, requires blobs. Yes, you do need to have blobs saved for this to work. This tutorial also works to install ios11.3.1 from ios11.3.1 beta 6. iOS11.3.1 beta 6 still signed so you can install via itunes and then use this tutorial to install the official iOS11.3.1 version:
Downgrade from any version to iOS 11.3.1 without losing data
Download iOS 11.3 beta 6 from The iPhone Wiki (still signed by apple)
Downgrade with Option + Update (Mac) or Shift + Update (Windows) in iTunes.
Install noncereboot1131UI, either the paid developer version or the free empty_list version.
Open your SHSH blobs in a text editor, and search for “generator”. Copy the value that follows it
Run noncereboot1131 and enter that value into the generator field, then hit enter. Reboot your device
Download futurerestore from encounter’s futurerestore fork
Run futurerestore as such:
./futurerestore -t [blob].shsh2 --latest-baseband --latest-sep [1131ipsw].ipsw -u -w
That’s it. Your device should update without losing data. If you get the message that the APNonce doesn’t match, you likely set your generator to the wrong value. If it fails, run
./futurerestore --exit-recoveryHow to update from ios 11 to ios 11.3.1 using shsh2