I'm having this very same issue. It's a bit frustrating, seeing that it is a new phone and that it immediately went into it post-activation. I've stopped it from updating the first time after it went through this:
5%…10%…25%…reboot…
0%…reboot..
Repeat.
I started the update again, and it's stuck in the reboot cycle again as I'm typing this.
Anybody here have a solution?
