If you don’t see an option to upload a file in the settings, your ISP has disabled manual flashing. They do this to ensure you don't install a generic TP-Link firmware that might break your VOIP settings or NBN configuration. How to Force an Update
Connect a device to your router (via Ethernet is best for stability). Open a web browser and type 192.168.1.1 in the address bar. vx420-g2h v2 firmware
Using a paperclip, hold the Reset button for 10 seconds. Note: This wipes your Wi-Fi passwords and settings, but it forces the modem to re-download the latest configuration and firmware from the ISP servers. If you don’t see an option to upload