I am out of space and I cannot upgrade my account now. What can I do to free some space? If I erase the uploads, would I lose all my data?

You can free up space by deleting the submissions. This will delete all the submission data for that submission.

Upload Space (total):  This limit refers to your forms that use a file upload field. The submissions can have file attachments on them and these files will take up the "Upload Space" limit. Note that this does not reset, this is fixed. If you wish to free up your upload space, you can delete submissions that have file uploads on them, but please do keep in mind that deleting submission data is irreversible and will permanently remove the data in our servers.

I suggest you download all the form data.

Complete Guide: How to Export All of Your Data at Once

After downloading the form data, you have a backup and you can delete the submissions.

You can also download the submissions as Excel/PDF/CSV and the Uploaded Files separately.

How to Download Form Submissions as Excel/CSV/PDF

How to Download the Submission Files