For example, I will update the firmware of the Asus WL-600g modem from version 1.0.3.3.A to 1.0.3.6A.
I will describe the actions:
1) Download the new firmware from the official site
https://www.asus.com/supportonly/WL-600g/HelpDesk_BIOS
2) Open the router settings by typing in the browser address http://192.168.1.1 (maybe 192.168.0.1) and enter the login/password.
3) In the interface that opens, at the bottom left, open the “Management” menu, select the “Update Software” tab, click “Browse” in the window that opens, select the firmware previously downloaded and unpacked from the archive, click “Send” to start the update process.
We are waiting for the completion of the process about 2-5 minutes.
Done.