Use internet speed over 15 Mbps (both download and upload) speed for error free update. Avoid latching on to Hotspot or WiFi Extenders
VPN or proxy connections might prevent your device from contacting the update servers.
If you still can't install the latest version of iOS, try downloading the update again:
If you don’t see the update in the list of apps or if the issue happens again, you need to update your device with iTunes.
Try updating iOS after each step.