How to delete submissions so I have more room?

You might want to download submissions first: 

https://www.jotform.com/help/73-How-to-Download-Form-Submissions-as-Excel-CSV-PDF 

https://www.jotform.com/help/374-How-to-export-all-of-your-data-at-once 

Then you can delete your submissions to free up storage: https://www.jotform.com/help/377-How-to-Delete-Form-Submission-Data