The reported issue effects iPhone 5s and earlier and iPad Air and earlier. The update requires an apple ID and password the finish the activation. People who don't know their passwords could not activate the device. Apple addressed the problem by pulling the update from older devices and replacing it with a different version of the update, in order to bypass the issue. If you phone is locked up by the update you can refer to the page below for a workaround on restetting the password. https://support.apple.com/en-us/HT206203
To follow up on what Sveta said, Apple pulled the original 9.3 update for the devices mentioned (iPhone 5S and earlier + iPad Air). However, a new build, 13E237, was just released that should work with those older devices.