Apple Releases Sixth iOS 10.3.3 + macOS 10.12.6 Betas, the beta version only includes new wallpapers for the 12.9-inch iPad Pro and no other visible changes, you could use 3uTools to upgrade or downgrade your iDevice to this version. 

Today 3uTools tests an iPhone 7 which is running iOS 11 beta2, we downgrade it to iOS 10.3.3beta 6 as reference.

Step1: Make sure that you have the latest version of 3uTools. Before upgrading, you should backup your iDevice. Connect your device to your computer, click the button ”Flash & Jailbreak", click the button “Easy flash”.  

 If you want to retain the data, please select the option " Retain User's Data While Flashing".

Step2: Select the firmware iOS 10.3.3beta6 , and then click the button “Flash”. (If you have downloaded the firmware, you need to import it to your PC first, and then flash your iPhone).

Step3: Keep a good connection between iDevice and PC, and then just wait until this process is finished.