When trying to update the IOM firmware from the CMC GUI you see the error below. I tried pulling the standby CMC and vice-versa but this did not work either.
Connect to the CMC management IP address via putty over SSH, and issue this command.
racadm fwupdate -s -m all
This should list all the modules that require a firmware update.
racadm fwupdate -u -m iominf-1
The command above will update the IOM is A1, so you will need to repeat this if you have more IOMs, changing the last digit. Be warned this command will reboot the switch!