How do I delete a Question Set within a Questionnaire?

To delete Question sets within a Questionnaire, navigate to the Questionnaires section of the Project, select the Actions button on the far right, and then select Edit from the Actions drop-down menu: 
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Once you are in the Questionnaire, navigate to the Question Set you wish to delete at the bottom of the Questionnaire, and then select the trash bin icon located to the far right that says Delete Question Set once you hover your cursor over it:

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When the page refreshes, you should have one less Question Set button at the bottom of the page.

Please note that if you've already mapped your questionnaire in Manage Criteria, you'll need to unmap it in order to delete the Question Set.