You want to restore a filesystem that you backed up by taking a snapshot.
First, reboot the router from the alternate boot media to which you saved the snapshot. On M-series and T-series routers, this is the hard disk:
aviva@RouterA> request system software reboot media disk
On J-series routers, this is either a removable compact flash card or a device connected to the router's USB port:
aviva@RouterA> request system software reboot media removable-compact-flash or aviva@RouterA> request system software reboot media usb
Then, retake the snapshot to copy the filesystem to the media from which you didn't boot:
aviva@router1> request system snapshot
aviva@RouterA> request system snapshot media removable-compact-flash
The procedure for returning to a snapshot filesystem backup is very straightforward. You boot the router from the alternate media and then run the snapshot command again, which copies the filesystem from the alternate media to the media from which you didn't boot.M-series and T-series routers normally boot from the internal flash drive, and the hard disk is the alternate boot media. J-series routers also normally boot from an internal flash drive, and the alternate boot media is a compact flash card that is either installed in the router's compact flash drive or in a USB device connected to the router.
Recipes 1.19 and 1.20
Router Configuration and File Management
Basic Router Security and Access Control
Routing Policy and Firewall Filters