Press and hold the button on the top of the iPad as indicated by the red arrow.
When you see this on the screen you can release the button on top of the iPad. Swipe across this message to turn the iPad off.
Tu turn the iPad back on press and hold the same button on the top of the iPad.
Continue to hold the button on the top of the iPad until you see this screen. You can release the button and the iPad will continue to boot up.