Once you have set Guided Access, if you want to stop it / exit the app, you have to enter the passcode that was originally set for Guided Access through the iPad Settings. 

 

If you have forgotten the passcode, here is how you can still get to reset the passcode: 

  1. Force shutdown the iPad by holding the power and home button for about 10 seconds until the screen blanks out and the Apple logo appears in the center of the screen. 
  2. This restarts the iPad and takes you to the main screen, after removing the Guided Access previously set. 
  3. Now you can go to Settings > Accessibility > Guided Access and change the passcode.


Also see : How to set up Guided Access in the iPad