Taking a snapshot
First, you need to take an archived snapshot of the current running application:
rhc snapshot save -a <app_name>
Restoring a snapshot
You have to create a new application using the same cartridges as the saved app, either on a different account or on the same after you delete your app using rhc app delete <app_name>. Do so by using the following:
rhc snapshot restore -a <app_name> -f <path/to/your/archive.tar.gz>
Note: the app name must be the same on both the old and new apps for the process to work
Note #2: you can run into problems restoring except with the most simple apps so be sure to have some other way of backing up the data on the app