Many apple users are facing the problem "Unable to Verify Update" while updating their iphones to ios11.Here in this article I am going to mention some workarounds you can try to solve this issue. Before you proceed, make sure that you are connected to the internet.
Shut down the Settings app
Firstly,shut down the Settings app. Double-tap the home button and then swipe up on the Settings until it slides out of view to uninstall it.
Tap Settings from the homescreen and go to General/Software Update. Try to install the update again and see if the issue is fixed.
Refresh your iPhone or iPad to refresh device memory. Try to update and check if this is resolved.
Reset your Network Settings
A network issue might be causing the problem, but you can try to reset these settings.
Go to Settings-> Reset->Reset Network Settings
and then select Reset. When your device has completed the reset,connect to
your Wi-Fi network and try to update again.
Delete the Update and Try Again
Sometimes a corrupt update can be causing a problem, you can delete that update and try to update again. Go to Settings->General->Storage & iCloud Usage
Tap on Manage Storage, scroll through it until you see this option "Software Update", select this entry and tap Delete Update.
You can now try to install the update again.
Delete the Beta profile
Go to Settings->General->Profile (you get the Profile menu if there are multiple profiles installed, such as joining the beta programme).
Tap the Beta Software Profile option and tap Delete to remove it. Then reboot your ipad or iphone.