Q: Backing up I phone 6s

If the iPhone 6S is truly new (just took it out of a factory sealed box), then there is nothing to back up!!

It depends on the amount of content on the phone, and whether you re backing up to iCloud or iTunes. However, it shouldn't take too long, since much of the content on the device is not backed up.

Slide to upgrade is not part of the backup. What exactly were you trying to do?

Are you in the midst of upgrading iOS?

If your iOS 9 device is stuck on Slide to Upgrade - Apple Support

Backing up the information TO iCloud, or restoring FROM iCloud? If you were just doing a backup, then you should not be getting a Slide to Update screen. Are you updating as sberman asked?

Sounds like they set the device up for you to restore from the backup, if this was a new device you just purchased. Your old phone was backed up to iCloud? Sounds like something about the restore went wrong. Try a reset on the device. Hold the sleep/wake and home buttons together until you see the Apple logo and then release. The phone should reboot and/or get you back to the setup screen.

Since "it won't do anything else", follow the instructions in the "Slide to Upgrade" article I attached above (here it is again):

If your iOS 9 device is stuck on Slide to Upgrade - Apple Support

Your new iPhone may have an older iOS version than your old one did.