iPhone 6 not updating to iOs 8.3 either on phone or itunes

I have tried several times to update my iPhone 6, both via the phone and iTunes, but it never completes.

On the phone I get to the agree to terms press it and then nothing happens it remains in the terms and conditions!

On iTunes it commences to down load but increases to 11 hours and never completes stating the download has timed out.

I have tried both several times to no effect. Phone currently at iOs 8.13.

I have even reset my phone totally and it still does not update!

Any suggestions?

Thanks Paul L