Says I have used all my upload space - How do I delete items I no longer need?

says i have used all my upload space.  how do I delete items i no longer need? 

You may free up some space by deleting submissions that have associated file uploads in it. However, be mindful that this process can not be undone.

Here's a snippet from the guide I'm linking below:

When deleting submissions individually, you will see this warning message (see screenshot below). Please note that deleting submission data is irreversible and will permanently remove it in our servers, including the file uploads on them.

Complete guide: How-to-Delete-Form-Submission-Data 

So I strongly urge you to make a backup copy of your Submission Data just in case you'd still need them. There are plenty of ways to do that, feel free pick one that will work for you:

Related guides:

How-to-Export-All-of-Your-Data-at-Once

How-to-Download-Form-Submissions-as-Excel-CSV-PDF

How-to-Download-the-Submission-Files

Otherwise, you can always upgrade to a paid plan to enjoy more storage for your file uploads.