Head over to the surveys page: https://app.surveyapp.io/surveys/ or click the Surveys tab on the main menu:
Locate your survey from the Surveys list, and then click on the three dots to reveal a drop down menu with additional actions.
Click on 'Delete this survey' in the menu. You will now be prompted to confirm your action:
Deleting a survey removes the survey and all collected data for that survey from the Surveyapp reporting system. You can't retrieve this data once this action is performed, so proceed with caution.
Click on 'Yes' to proceed with the deletion.