How to turn off guided access without home button

It is a common misconception that an iPad (or iPhone) can remain fully functional with damaged or defective physical buttons. 

Whilst Accessibility features (such as assistive touch) can be used to augment the User Interface, these features cannot be used as a substitute for physical button sequences that must be used to trigger system recovery actions. Sooner or later, as now, you will encounter a situation from which you cannot recover.

Until repaired, you will not be able to use your iPad:

iPad Repair & Service - Apple Support

When you are stuck in Guided Access mode, you are really stuck as the typical triple tap on the home button to quit out of Guided Access doesn't work. Even a forced exit won't work because the power button and the home button are disabled. 

The only way to get out of Guided Access is to press the Home and Power button together for 15 seconds. This should turn off Guided Access by forcibly rebooting your device. Once your device is restarted, you can then go to Settings > Accessibility > Guided Access > Guided Access - and turn off Guided Access if needed.

Use Guided Access on iPad - Apple Support

Use Guided Access with iPhone, iPad, and iPod touch - Apple Support

Make an appointment at the Genius Bar and get it fixed.