There are two options:
- Delete all responses.
- Delete all incomplete responses.
The deleted responses will be deleted from the database, and is not recoverable. Respondents will not be able to answer the survey during deletion as both these actions will temporary lock the survey. If you want to edit the structure of the survey (e.g. add/edit questions) you must delete all responses first, if any. Another alternative is to make a copy of the survey, make the changes, and then publish this one instead.