Q: how to update ios8

You should, if you have enough free space on the phone (and you aren't blocking the update servers), be able to update directly on the phone via Settings > General > Software Update,  or via the Summary tab when connected to your computer's iTunes - after taking a backup, copying any purchases over to your computer's iTunes library (File > Devices > Transfer Purchases), and preferably taking your own copy of any important photos and/or documents.

If that isn't working then what happens, if you are getting an error message then what does it say ?

(I've asked for your post to be moved to the Using iPhone forum, where you've posted is for questions about the iTunes U app.)