I can’t activate my iPhone6s at all after updating it

Try the following steps

• connect phone to computer with the latest iTunes installed
• while phone is connected, hold down the home button and the power button until you see the connect to iTunes screen
• when you see the option to restore or update on your computer, choose update.
• Wait until iTunes downloads the software for your device. If the download takes more than 15 minutes your phone will exit the connect to iTunes screen, let the download finish, then repeat steps 2 and 3

You should now be able to use your device as normal