There isn't an unpublish function, but there are alternatives.
If you want to unpublish an app with the intent to never publish it again, you can just delete the app.
If you want to temporarily unpublish an app, you can just rename it to something else.
To delete or rename, left click on the app name in the navigator on the builder tab.
If neither of these options work for you, open a support ticket with the name of the app and we can manually unpublish the app for you.