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You maybe mean delete the "Uploaded Files" which sent to you via a form submitted to you, to free up space
If that's what you mean then follow this steps:
1.Click On "My Forms" (Link: http://jotform.com/myforms)
2. Select the Forms which the files sent to you through..
3. Click on "Submissions" on the top of the page and delete all of it after backup those files on your computer.
As mentioned by MohamedRIYAD above, that is correct.
I would only like to add that once removed, there is no way to restore it, which is why it is good to make a backup first if you will need that data at some later point as well.
Another thing to mention is that you would only need to remove the submissions that have some files uploaded to them, since these are the ones taking a lot of space, while the plain submissions (without any files) do not take a lot of space (hundreds of such could take as much as 1 with a single file).
If you happen to have any additional questions, do let us know and we would be happy to assist.