Router#upgrade rom-monitor file tftp://192.168.30.10/C2800NM_RM2.srec.124-13r.T11
You should change C2800NM_RM2.srec.124-13r.T11 to the actual file name that you downloaded from Cisco. Execute the command, the router will download the image and prompt you to allow it to reboot.
Router#upgrade rom-monitor file tftp://192.168.30.10/C2800NM_RM2.srec.124-13r.T11
Loading C2800NM_RM2.srec.124-13r.T11 from 192.168.30.10 (via
GigabitEthernet0/0): !!!!!!!
[OK - 1626367 bytes]
This command will result in a 'power-on reset' of the router!
Continue? [yes/no]: yes
System configuration has been modified. Save? [yes/no]: no
ROMMON image upgrade in progress.
Erasing boot flash
eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee
Programming boot flash ppppppppppp
Now Reloading
Select Yes and the router will erase the current boot flash and program the new image and reload.