What version you are at ? You cannot download ISO images of OSes for a new install. But if you are at a version like 2000.2.x or 2000.4.x you can download the OS ugprade files and then run the executable on the server. Once you run the OS upgrade, then you can run the OS service releases for the current version of OS upgrade that you installed. For ex: if you are at 2000.4.2 and you run executable for OS upgrade 2000.4.3. Once this upgrade is complete (server reboots twice) then you can download any 2000.4.3 OS service releases (for ex: 2000.4.3 sr7). This will also cause server to reboot mostly once.
HTH
Sankar
PS: please remember to rate posts!
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus