As an example, I will use the RT-N10LX router with firmware version 1.0.0.6.
First of all, see if there is a newer firmware on the official website:
https://www.asus.com/Networking/RTN10LX/HelpDesk_BIOS/
Download the archive with the new firmware and unpack it.
Open the web-based interface of the router by typing the address http://192.168.1.1 in the browser and enter the login – admin, password – admin, and look at the firmware version that is installed, it is usually written at the very top, for example, “Firmware version: 1.0.0.6”.
Next, open the “Administration” menu, select the “Firmware Update” tab, click “Browse” and select the firmware file downloaded and unpacked from the archive (in my case it is RT-N10LX_2.0.0.37.trx), click “Submit” and wait for the update process to complete.
The configuration of the router after updating the firmware remained unchanged.
If a failure occurs during the upgrade, the router can be restored using a utility that can be downloaded from the official site using the link above.