how to clear submissions

Do you want to delete the existing submissions of your JotForm? You can delete the submissions from the submissions view. 

http://www.jotform.com/help/269-How-to-view-Submissions

You can delete individual submission by clicking on X mark on each submission. If you want to delete multiple submissions, you can hold Ctrl key on the keyboard and select multiple submissions. Clicking on "Delete all Submissions" would delete all the submissions of the JotForm. Please be noted that the submissions cannot be recovered in anyway once they are deleted from JotForm. You'll need to make sure that you have a backup of submissions. You may integrate Google drive, Dropbox, Google Spreadsheet, etc apps as a backup.

Hope this information helps!