Click on the Folder you want to modify. You can modify the name of any Folder you created, on your My Designs page, in Team Designs or in a Group.

Click the down arrow next to the Folder name at the top of the page.

Click "Rename" and type a new Folder name into the modal (box) that appears, then click the "Rename" button to confirm (or "Cancel" if you want to leave the Folder name as-is).