How do I get back to the 'Set up iPhone' screen?

Go to Settings>General>Reset>Erase all content and settings that will make wverything dissapear and get you bach to the setup screen. Make an iTunes backup on your old device for the data then resroee that to your new iPhone.

i would make a backup if you can then restore your iPhone use the third way on the following link

If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

this will reset it to factory settings and get you back to the setup screen

I can't start up my iphone , help please??

Settings

General

Reset

Erase all content and settings

This will delete everything off your device, so then you can restart your setup and restore from a backup!

Hi there,

You may find the article below helpful.

iCloud: Restore your iOS device from iCloud

http://support.apple.com/kb/ph12521

-Griff W.