For example, I will flash the Asus DSL-N14U modem with firmware 126.96.36.199 for version 188.8.131.52.
I will describe the steps taken in steps:
1) Download the new firmware from the official site
2) Open the router settings by typing in the browser address http://192.168.1.1 (maybe 192.168.0.1) and enter login/password (standard admin/admin).
3) In the interface that opens, at the bottom left, open the menu “Administration“, select the tab “Firmware Update“, in the opened window, click the “Check” button, the modem itself will check for a new firmware version, if it exists, then a notification will appear “A new firmware version is available. Do you want to update the firmware?“, click “OK” for the agreement. If the modem cannot perform the check, then click “Browse” and 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.