For example, I will update the firmware of ASUS RT-N12LX router from version 220.127.116.11 to 18.104.22.168.
I will describe the steps in steps:
1) Download the new firmware from the official site
2) Open the settings of the router by typing the address http://192.168.0.1 in the browser (maybe 192.168.1.1) and enter the standard login – admin, the password – admin
3) In the interface that opens at the bottom left, open the Administration menu, select the Firmware Update tab, click Browse in the window that opens and select the firmware that you previously downloaded and unpacked from the archive, and click Submit to start the update process. We are waiting for the completion of the process about 2-5 minutes.