You can edit a device group to: add Unassigned Firewall(s) to the group; remove firewalls from the group; update friendly/group names and tags.
To edit a device group:
NOTE: The Root Group cannot be edited.
1. Navigate to Manager View | Firewalls > Groups.
2. In the Action field for the group you want to edit, select Edit Device Group. 
3.Make changes to the Group Name and Tags fields, if needed. 
4. To add devices to the group, select devices in the Unassigned Devices list and click the caret-right icon to move them to the In Group. To remove devices from the group, select the devices in In Group list and click the left-caret icon to move the devices to the Unassigned Devices list.
5. Click Save.
How to create device groups in NSM: https://www.sonicwall.com/support/knowledge-base/creating-device-groups-in-nsm/23110800023087