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Spectrum will push later firmware version provided by the gateway manufacturer when it fixes a device problem they are experiencing on their network, after they have verified the new code causes no harm to the cable network proper, nor to other subscribers' equipment.
EDIT: If you buy your own router with WiFi and connect to Spectrum's modem, you DO get firmware upgrade support from Netgear. Only customer-purchased modem or combo-gateway firmware is distributed by Spectrum.