Never upgrade firmware over Wi-Fi. If the wireless signal drops during the upload, the router could become unusable. Use an Ethernet cable (RJ45) to connect your laptop directly to one of the LAN ports.
If the "Software Update" tab is missing or greyed out (common on ISP-locked units), you may need to access the or use a tool like TFTP . zte f670l upgrade firmware portable
Updating firmware carries a small risk; if interrupted, it can "brick" your router. Never upgrade firmware over Wi-Fi