For now all info on this subject is here
In order to restore the data from a snapshot, you replace the ncdata subvolume, as for example:
To remove datadir
sudo btrfs subvolume delete /media/USBdrive/ncdata
To restore old copy
sudo btrfs subvolume snapshot /media/USBdrive/ncp-snapshots/daily-xxxxx /media/USBdrive/ncdata
To update NC database with the new/old files
nc-scan