Am I able to delete submissions to stay within the limit of 500?

Yes, you can follow the instructions in this guide to delete the submissions: https://www.jotform.com/help/377-How-to-Delete-Form-Submission-Data 

Before deleting I would recommend, you to download the submissions first: How-to-Download-Form-Submissions-as-Excel-CSV-PDF 

Please review and let us know if you have any further questions.