Update router firmware?

I have a Netgear CG4500BD modem/router purchased from Cox.  How can I check to see whether the firmware is up to date, and update it if necessary?  Cox tells me that since I bought the modem I have to ask Netgear how to update it.  Netgear says that since the modem was purchased from Cox, it can only be updated by Cox.  Any help appreciated.