Q: help! I just bought an iPhone 6 s and tried to sync to my macbook and it won't let me "slide to upgrade"...

Hi! Take a look at this article.

If the Slide to Upgrade screen won't go away after you update to iOS 9 - Apple Support

If this doesn't help, try the steps below.

Try to reset your device. This won't delete your data stored on this device.

• Press and hold the Sleep/Wake button
• Press and hold on the Home button
• Keep holding both buttons until the display turns off and back on with Apple logo on it.

Alternatively, you may go to Settings - General - Reset - Reset All Settings

https://support.apple.com/en-us/HT201559

As a final step, try the recovery mode. Turn your device off, then plug it to your computer with the home button pressed. Keep the Home button pressed until you see the iTunes logo on the screen of your device. After that on your computer you should see the iTunes window saying your device needs to be restored to factory settings. Click on the restore button.

More info here:

https://support.apple.com/en-us/HT201263

Also, try updating your phone to iOS 9.1, this is the latest version of iOS.