How can I free up some storage space?

where is the data storege button (100 mb free) which i manage to delete files if required

There's no button per se to delete files then free up space. What you need to do is delete 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