how do i turn off an ipad air that has one button

See this article:--> Restart your iPad - Apple Support

and/or use these instructions to "force" restart your device:

• On an iPad with a Home button: Press and hold the top (power) button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On other iPad models: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top (power) button. Only when the Apple logo appears (it might take 15-30 seconds), release that button (ignore all other screens that might appear).

If the issue persists, contact Apple Support, using this link:--> Get Support

Regards

Giulio

See >>> Turn iPad off or on – Apple Support