Open a browser and type 192.168.1.1 (or the gateway IP on the back label).
Check the label on the device. It may say: vn020-f3 firmware
Before updating, you need to know your current version to ensure you aren't "downgrading" or reinstalling the same software. Connect your computer to the router via Ethernet. Open a browser and type 192
However, if you are referring to firmware for a (e.g., a router, ONT, or modem) with a model number containing “VN020” — for instance, the Calix VN020 (a GPON ONT used by some ISPs) — I can offer a general safe firmware guide for such devices. vn020-f3 firmware