Hey,
I'm not quite sure about the update part.
But there are a few reasons why you might be only at 28.8.
1- Your isp dosnt support greater
2- Current network conditions, peak hours?
3- you have bad settings in your PC.
I can't help for 1-2 but I'll give you a few things to check for #3.
1- in your dial-up properties, server type, and uncheck all ticks except tcp/ip
2- modem properties, Maximum Speed, select the highest
3- modem properties, connection, port settings, put receive buffer to the max.
4- modem properties, connection, advanced, disable error control
This should improve your connection speed a little.
Keep in mind that the general 56k modem user will connect at 36.6 or lower, even with an updated modem.