This cannot be done manually! Microsoft says:
After a user enrolls a phone for company app distribution manually by tapping an AET.aetx file on their phone, the phone is automatically enrolled for as long as the certificate is valid (one year). After enrolling for company app distribution by this process, users cannot unenroll their phone by using the phone UI.
Source: http://msdn.microsoft.com/en-us/library/windowsphone/develop/jj206943%28v=vs.105%29.aspx
However if you use the MDM Api to enroll a Phone (like Intune does), and unenroll that phone (either remote or manually) the aet is removed, too.