Just had that problem like 15 mins ago but yeah, usually for a fix it would be these:
- going onto itunes from the pc and plug in his device > preference (parental control) uncheck the allow to go to app store then save>exit itunes from computer reboot his device then relaunch itunes from pc again and check the allow to access the app store then save and see if it would work.
- other than that the other method would be set the time and date 1 day advance then set it to automatic so that it changes it back and it should work.
- if that doesn't either then just go to wifi (blue arrow to manual set up) and under dns delete all those numbers of the address until you are left with 8.8.8.8
- last thing you want to do is a system restore, however if you are jailbroken then use the cydia impactor to regain that current ios instead of an update from the restore, but.. you're not so these are your only choices
it may be a server sided issue from apple, i was able to get in from the "cannot connect to itunes" after logging out because of that fail update etc. try logging out of your account with the same apple id from all your other devices and log in 1 at a time (if you have automatic download to device app from your apple id from different devices be sure to turn that off as it may be also that is the problem).
hope this helps, give a thanks button or rep if it did