Apple on Monday released the first public beta of iOS 11, offering iPhone and iPad with extra test devices the opportunity to sample the upcoming operating system prior to its debut this fall. You could flash the firmware in 3uTools.

Compatibility list of iOS 11 beta2 update1:

iPhone SE, iPhone 5s, iPhone 6, iPhone 6s, iPhone 6s Plus, iPhone 6 Plus;

iPad mini 4, iPad mini 3, iPad mini 2, iPad Pro (9.7‑inch), iPad Pro (12.9‑inch), iPad Air 2, iPad Air.

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 11 beta2 , 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. 

Proper iOS 11 Downgrade Procedure Detailed on Apple Support Page, you could downgrade to iOS 10.3.3 or other available iOS by iTunes and 3uTools.