Using the example, I will update the firmware in the old ASUS WL-520GU router.
At the time of writing, the newest firmware was 18.104.22.168 for 2012.
I will describe the steps in steps:
1) Download the new firmware from the official site
2) Let’s open the router settings by typing in the browser the address http://192.168.0.1 (maybe 192.168.1.1) and enter the standard login – admin, password – admin
3) In the interface that opens, on the bottom left, open the “Administration” menu, select the “Firmware Update” 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 for about 2-5 minutes.