Make sure that you are not missing something 😉 Turn it off and then turn it back on. At the Chuwi logo press the VOL- button for 10 secs. It should take you to the OS choosing menu. Or if you are in the Android OS check the notification bar if there is a Change to Windows button.
Unfortunately I did not find a guide if you really do not have Windows installed, but I will check back if I find anything.