I think there is a problem currently. Checked a couple of free apps myself and none have backups. It seems that opening the app is alos a problem because all I get is 404 Not Found: Requested route errors. File a support ticket. Mendix should fix this.
Hi Molly, greeting from future, After a year :)
On your computer, there is a file of your own app in the mendix file in the documents, you can copy this file and store it in another place. After you're done with your changes of application, you can paste this backup file back in the same place and bring everything back!
I hope you solved that 1 year ago / it will help now! :)