Generally speaking from a consumer's point-of-view -- no, there isn't any way to flash your modem 'manually'. Your cable modem is controlled by the ISP you're connected to - so, in essence, they can leave the current firmware loaded from the manufacturer, or load a firmware from their head-end to each cable modem on the network.
Can you call for an update ... if available? Yes, sure; there's no harm in calling. They'd probably tell you that you have the latest one the ISP is working with at this point. If there is any latest firmware - the CMTS will probably flash your cable modem automatically, which will likely re-boot your cable modem; normal operation of flashing the firmware.