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I checked your forms and noticed that you have 3 forms with uploaded files, this is why you almost reach the limit, these are the forms:
You will need to delete the submissions in order to get free space, I would suggest you to download all the uploaded files, while on the submissions page click on the "Download all files":
Also, download all the submissions data, this guide will help you with that: http://www.jotform.com/help/73-How-to-Partially-Download-Form-Submissions-as-Excel-CSV-PDF.
I would suggest you to integrate your form with a 3rd party service in order to get your uploaded files there, you may use either Dropbox or Google Drive, so you will receive your uploaded files and submission data on your 3rd party account.