If you get an Appstore ERROR or are not able to download an update from the Appstore you can try the below steps.
Step 01. Remove all data & cache from safari
– go to settings > safari > clear history and website data
Step 02. Reset the network settings on your mobile devices
– go to settings > general > reset > reset network settings
Step 03. Sign out from the app store
– go to settings > itunes & app store > tap your apple id > tap sign out > sign in with your existing id.