Please refer my
previous article to learn more about creating NINTEX forms in SharePoint 2013 Online.
I have created a NINTEX form in the custom list and I need to revert back to default SharePoint form instead of using NINTEX form. Navigate to the custom list where the NINTEX form is created. Click on List tab and then click on NINTEX Forms button in the ribbon interface.
Click on Delete Form in the interface so that the default SharePoint form can be used.