Apple today seeded the golden master (GM) version of iOS 11 to developers. Check Apple Seeds iOS 11 Golden Master to Developers Ahead of September 19 Public Launch for more details. 

Here, iOS 11 GM is available on 3uTools, you can now install easily. If you don't know much about GM version, please check What’ s Golden Master (GM) ?

Step 1 Launch the latest version of 3uTools and then connect your iDevice to your PC. 

Step 2 Click Flash & JB -> Easy Flash, select iOS 11 GM firmware as the following picture. Click "Flash" to start the process. ( If you want to save data, just select the option "Retain User's Data While Flashing" before flash.)

Step 3 Keep a good connection and internet while flashing. After a few minutes, your iDevice will automatically reboot. Now you can experience iOS 11 GM, although it was leaked before.