I am upgrading the Cisco 2851 router rommon from Version 12.4(1r) to recent release C2800NM_RM2.srec.124-13r.T6.
I searched the entire Cisco but couldn't find proper one.
Please anyone help me providing me the upgradation procedure (is it through TFTP or using USB).
You have the correct amount RAM for the IOS that you are using now and the IOS that you plan to use. The problem that you are facing is the amount of flash memory onboard for 2 IOS images. In your case you have to delete the IOS that you are currently using to make room for the new IOS.