Here is the procedure on how to downgrade iOS 11 beta version to current working version i.e. 9.3.x. Now you can try the iOS 11 beta version to enjoy its latest features that shall be finally released this year very soon. Steps are given for downgrading iOS 11 beta because Apple is currently signing the 9.3.x versions and it is not very difficult for users to get back to older version while testing the iOS 11.
Obviously it is understood that once you have downgraded you shall be able to restore using your previous backup or set your device as a new one. If you have made any backup in iOS 11 that will not be available in 9.3.x versions.
Before starting the process please make sure that your computer is having latest version of iTunes which you can upgrade once it gives the prompt for new version because the restore or downgrade process requires latest version of iTUnes.
Secondly visit our iOS download page and download the latest ipsw file which is compatible to your device which you are going to downgrade.
Next step is to turn off the option of Find My iPhone feature. Navigate to Settings => iCloud => Find My iPhone. Toggle the Find My iPhone switch to the Off position and authenticate with the iCloud password when prompted.
Put Your Device in DFU / Recovery Mode
Now connect your device to computer and iTunes should launch automatically. Wait a while and put your device in DFU mode also called recovery mode. If you are not having iPhone 7 you can hold down the Power and Home button for 10 seconds, then release the Power button but ensure that Home button is not released. You will see that device is in DFU mode and iTunes provides an alert saying that it has detected a device in Recovery Mode.
For iphone 7 and 7 Plus follow below method to put in DFU or recovery mode:
Turn off the phone => Press and hold the power/lock button for 3 seconds => With the power/lock button still pressed, press and hold the volume down button on the other side of the iPhone => Keep holding both buttons down together for approximately 10 seconds => After approximately 10 seconds, release the power/lock button but continue to keep pressing the volume down button for another 5 seconds. You will see prompt in iTunes that device is in DFU mode.
If wrong combination is pressed or buttons are pressed for longer period you will either see Apple logo or iTunes logo with lightning cable on screen. In this case you need to turn off the mobile and try again.
Once you have put the device in recovery mode on your computer in iTunes window you have to click the Restore iPhone button while pressing and holding the Option key (Mac), or SHIFT key (Windows). After the click a prompt will ask you to chose the ipsw file for iOS 10.3.x which you already stored on your computer. This is the start of restoring or downgrading process and remaining process iTunes complete with due time. Just relax and wait for the process to complete. Now your device will restart and you are able to set it up with your favorite settings.