It is controlled by the ISP. I work for my ISP
But seriously, you can go on the forums at DSLReports, find your ISP's boards, find the list of approved employees (should be a stickey) and PM them. Usually all they need is your MAC address(AKA your HFC MAC address) which can be found on a sticker on the bottom of your modem. If your ISP does not have any offical employees on DSLReports, then you can call your ISP and ask them. But the adverage Rep won't have any idea what your talking about and/or won't have access to upgrade it. You can ask for Tier2 or something, but your better off checking their support site and trying to find a email. You usally get better answers from people who aren't trying to keep down their adverage handle time. Hehe.
Also, it should be noted that usually the most up to date "approved" firmware gets loaded when you start your modem. I say usually because sometimes it doesn't. Also, approved doesn't mean newest. There is a whole certification process between the manufacture releasing it and the ISP adapting it to their network and testing it. And firmware that kicks arse in one market of one ISP, might suck for another ISP, or even for a different market(town,state,etc) for that same ISP.