just call me, and we will increase the speed.
In the world of cable modems, a single channel carries ~39Mbps down and ~10Mbps up. That is the physical signaling rate -- much like ethernet commonly supports 10, 100, and 1000. The DOCSIS configuration sent to the modem during startup tells it, among other things, the user rate for both up an down. The modem can only really police upstream, the network handles downstream. As such, the quickest, simplest, and most common method for "increas[ing] the speed" is to change the numbers in the config file.
The other, less commonly used, path is to move the modem to different upstream and/or downstream channels. This is usually an automatic traffic engineering process on the CMTS ("headend") to balance load across channels (i.e. minimize congestion), but it is still a configurable item. Changing channels can also move you to a less noisy channel; cable plants are dynamic things and some channels will have more/less noise (or signal) than others.
** DOCSIS 3.0 introduces channel bonding. As such, the modem uses all the available channels at once -- there's no channel to move to/from as it will already attempt to use all of them. That said, may US providers have not started bonding upstream, yet, so that can still be manipulated.