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If you meant the upload space you have accumulated on your account submission data, you can decrease/clear them by deleting submissions that have upload submissions on them.
You can go to the form submission page http://www.jotform.com/help/269-How-to-view-Submissions
and delete the submissions that have uploads.
Please take Note: Deleted submissions are permanently gone. So, make sure to have a backup copy of the submission before deleting them.
Let us know if you need further assistance.