since your question was not relevant for the topic where you posted it, I started a new topic.
Downgrading to iOS10 is something I would not recommend, unless you really really need to.
The best way to do this is through iTunes.
I found these two links where they show how it's done (link1, link2). I have no idea if this still can be done, there are several warnings about loosing your contacts and/or firmware files no longer being available, but here are the instructions:
How to Downgrade iOS 11 to iOS 10.3.3 the Easy Way
Be sure you have recent backups available of your iOS device to avoid potential data loss. An iOS 11 backup is only able to be restored to an iOS 11 device, thus you would need an iOS 10 backup to restore to iOS 10. Failing to have a compatible backup can lead to permanent data loss.
- Go here and download the iOS 10.3.3 IPSW file for the iPhone or iPad you wish to downgrade and save that .ipsw file somewhere easy to locate like the desktop. (scroll down and find the file that is suitable for your device)
- Open the iTunes app on a Mac or Windows PC.
- Using a USB cable, connect the iPhone, iPad, or iPod touch you wish to downgrade to the computer.
- Select the device in iTunes by clicking on the little device icon in the upper left corner of iTunes.
- Now under the device summary section of iTunes, click the ‘Update’ button using the appropriate modifier key to begin the downgrade process:
- Mac OS: OPTION + click the “Update” button
- Windows PC: SHIFT + click the “Update” button
- Select the iOS 10.3.3 .ipsw file you downloaded in the first step, and choose to update to that version.
- The iPhone or iPad screen will turn black as the downgrade begins, rebooting multiple times with a progress bar and Apple logo as the downgrade completes.
- When the downgrade has finished the device will boot back up into iOS 10, the process can take a while if the device has a large amount of used storage.
The downgrade from iOS 11 to iOS 10.3.3 should go without a hitch, assuming you chose the proper iOS 10.3.3 IPSW file for your device, and assuming Apple is still signing the system software. Once Apple stops signing iOS 10.3.3 then downgrading to it from iOS 11 will be impossible.
Some users have reported that sometimes data in iBooks, Notes, Music, and Messages may go missing with the “update” downgrade approach outlined above. If that happens to you, simply restore the device from an iOS 10.3.3 compatible backup once the device is back on iOS 10.3.3.
Note that if you choose “Restore” in the above process and then select IPSW, then the device will be either restored from a backup made compatible with iOS 10.3.3 or setup as new like a standard restore process, which can then be restored with a compatible backup.
If for some reason the above downgrade method fails, you can use the Recovery Mode approach to downgrading iOS 11 detailed next.