update to 8.4.1 please help

Hi there

i got the notification to update to iso 8.4.1, tried to do it many time via the phone and it keeps giving an error message.

now i am trying to to do it via the itunes, it started then is gives me an error -18 message. i updated my computer, and my itunes are up to date.

Please help me my phone is totally off and requiring to plug to itunes