Q: my 5s wont let me "swipe to update" after installing ios9

I've opted to upgrade to ios9 on my 5s. I had not previously updated to ios 8. My phone is now stuck on the "swipe to upgrade" screen and I can't get past it.

I have held down the home and lock button to restart, which took then asked for my passcode and allowed me to select a wifi network before returning to the same white "swipe to upgrade" screen.

Please help?