Paying for preferred - only getting 9.5Mb/s

So i recently noticed that i was paying for the cox preferred internet package, but I was only consistently getting a downstream speed of 9.5Mb/s but am still getting the advertised upstream speed of 5.0 Mb/s (using both Cox's tool and speedtest to check this). I'm not sure if this is just because I have had this plan forever and Cox never updated my speed according to their advertised speed, or if it is an issue with my modem. I have a fairly old wired SB5101 Motorola surfboard modem, but from what I understand about networking standards, the older standard is fine up to 38Mb/s speeds. Unless it is possible that a firmware update is required to receive the advertised 25Mb/s speeds, I am not sure what the issue is.