1: Launch the Settings app, scroll down and tap on iTunes & App Store.
2: Tap on your Apple ID, and select the ‘Sign Out’ option from the pop-up menu.
3: Type the password to your Apple ID to login again. Test.
Take a look at this Apple Support article:
https://support.apple.com/en-us/HT204051
Reset your network settings.
Settings - General - Reset - Reset Network Settings
Reset your device. This won't delete your data stored on the phone.
- Press and hold the Sleep/Wake button
- Press and hold on the Home button
- Keep holding both buttons until the display turns off and back on with Apple logo on it.
Alternatively, you may go to Settings - General - Reset - Reset All Settings
https://support.apple.com/en-us/HT201559