Apple will be rolling out the highly-anticipated iOS 10 update on 13 September around 10am PST while avid iOS fans will be eager to download the new firmware in a hurry. As this is the first public release of iOS 10, it may harbour some unexpected bugs or broken features, which might force you to downgrade it to the latest digitally signed version of iOS 9 or iOS 9.3.5.
If you find any performance problems such as rapid battery-drain or slow-app-loading issues with iOS 10, you could quickly rollback to an older firmware release, using the step-by-step guide posted below. As a word of caution, avid iOS users are advised to hold back the temptation of installing iOS 10 on the launch day and instead wait for initial feedbacks from the iOS community regarding potential bugs and issues in the new firmware.
iOS 9.3.5 download IPSW links:
Steps to downgrade iOS 10 to iOS 9.x on iPhone, iPad or iPod Touch
Step 1: Ensure you have installed the latest version of iTunes on your Mac or Windows PC.
Step 2: Download iOS 9.3.5 IPSW file from the links provided (above) for your specific model of iPhone, iPad or iPod touch.
Step 3: Disable the Find My iPhone feature on your device. Go to Settings > iCloud > Find My iPhone, and verify that the option is toggled off.
Step 4: Connect the iOS device to your Mac or PC via USB cable and launch iTunes.
Step 5: Boot the device into DFU mode by holding down both Power and Home/Touch ID buttons on the device for 10 seconds. Release the Power button but continue holding the Home/Touch ID button until the device enters DFU mode. An alert should pop up in iTunes notifying you that the device is now in Recovery Mode.
Step 6: When your device appears to be successfully recognised in iTunes, press the Option key on the Mac or the SHIFT key on a Windows machine, and then hit the "Restore iPhone..." button.
Step 7: When asked, browse and select the iOS 9.3.5 firmware file that was downloaded earlier. iTunes will now go through the process of installing a fresh version of iOS 9.3.5 to your device.
Step 8: Do not disconnect the device from the computer until iTunes confirms that the restore process is complete or the device has rebooted into the welcome screen.
Step 9: You can now restore your backup files for iOS 9.3.5 or older firmware that you created earlier. Any data backups taken in iOS 10 will not work after you downgrade back to iOS 9.3.5.