Firmware update for user owned SB6141?

I have my own SB 6141 modem I'm using on my COX service. I'm getting slow speeds which I'm suspecting old firmware on the modem since I've had the unit awhile. Contacted COX and they say they can't update firmware and that I need to do the update myself. My understanding is that the service provider always controls the firmware releases on hardware so that they don't have issues. How can I go about upgrading to the right firmware? Sounds like even if i bought a newer model modem it still wouldn't work if they aren't willing to release the current firmware updates.