As an example, I will update the ASUS RT-N13U router.
I had at hand an ASUS RT-N13U Rev.B1 with firmware 2.0.2.0.
1) First of all, open the web-interface of the router by typing its IP address http://192.168.1.1 in the browser and enter the login – admin, password – admin, look at the firmware version that is installed, usually it is written at the very top, for example “Firmware version: 2.0.2.0”.
2) Let’s see if there are new firmware versions on the official website:
https://www.asus.com/Networking/RTN13U/HelpDesk_BIOS/
https://www.asus.com/Networking/RTN13U_B1/HelpDesk_BIOS/
In my case, a new version 2.0.2.5p was found, I downloaded the archive with this firmware and unpacked it. If you have a router revision A1, then it is absolutely impossible to install the firmware from B1 on it, as it will damage it.
3) Next, you need to open the “Administration” menu, select the “Firmware update” tab, click “Browse” and select the previously downloaded and unpacked firmware file from the archive, click “Send” and wait for the update process to complete.
When the firmware update process is in progress, you must absolutely not disconnect the power supply from the device. From the links above, you can also download a utility to restore the router, in case it does not start due to damaged firmware.