You can downgrade from the iOS 10 beta to iOS 9.3.x fairly easily, but you need to take one special step during the restore process or your downgrade to iOS 9 will fail.
Using this guide you can downgrade from iOS 10 to iOS 9.3.x from the developer iOS 10 beta and it should work later this year to downgrade from the public iOS 10 beta to iOS9.3.x
Backup using iTunes or iCloud before Proceed. It’s always better to be on the safer side.
Step 1 : Go to this link and download the .ipsw (iPhone Update Software) for your iPhone, iPod touch, iPad.
Step 2 : Open iTunes on your mac / pc and connect your iPhone, iPod Touch, iPad
Step 3 : Click on the Summary tab below the device thumbnail. Then click on the “Check for Update” by holding the option key on mac / shift key on pc. A new window pops up.
Step 4 : Select the .ipsw file we downloaded from the link given at first. Click Open
Within 10 to 20 minutes your device will be downgraded to 9.3.x