If you can‘t activate your iPhone, iPad, or iPod touch after installing a software update. Apple’s official recommendations
After you update, you might see the Activation Lock screen on your iPhone, iPad, or iPod touch. If you enter your Apple ID and password and still can’t sign in, try these steps.
Updating some iOS devices (iPhone 5s and earlier and iPad Air and earlier) to iOS 9.3 can require entering the Apple ID and password used to set up the device in order to complete the software update. If you can’t recall your password or can’t sign in, try these steps.
- Sign in to iCloud.com from a computer. You can get help if you forgot your Apple ID or need to reset your Apple ID password.
- Open Settings and scroll to the My Devices section.
- Confirm that your device is listed in the My Devices section. If you don’t see it listed, check all your accounts.
- Connect your iPhone, iPad, or iPod touch to the computer.
- Open iTunes. Make sure that you have the latest version of iTunes.
- Select your device. You should see the Activate [your device] screen in iTunes.
- Enter your Apple ID and password and click Continue.
Or try to sign in to iCloud.com, so you can remove Activation Lock:
- Turn off your device or take it offline.
- Sign in to iCloud.com with your Apple ID and password.
- Open Find My iPhone.
- Select All Devices, then select your device.
- Click Remove from Account. If you still see a map in the device detail view, select All Devices again, click next to your device, then click Remove.
- After you set up your device successfully, make sure that you sign in to iCloud again.
Source is Apple Support.