Teenu,
If you are using MDDT, there are actually no commands. The procedure is something like this:
Open MDDT, check that you have your system in the List Of Customers. If not, goto Tools - CDA, create the customer (your system) with username, PSW, IP address. Exit CDA, refresh the customer info in Tools (MDDT).
Then connect to the system. Goto USR1 (system side on the right) - SES - DR - REL1. Copy REL1 to the desired directory (folder) in your PC (left arrow in the middle). If needed, do the same with USR2-SES-DR-REL2.
Done!
If you want to copy it back, follow the procedure described, but vice versa.
If the system is totally down (no contact to NIU or something), you need to have an extra flash with a normal dump and insert it in the NIU2.
Good luck