As an example, I will use the TP-LINK TL-MR3420 V1 router with firmware 3.12.19 Build 110617
I will describe the steps in steps:
1) Download the new firmware from the official website
There are firmware for the versions of routers V1 and V2. You need to download the firmware for the version of the router that is written on the sticker under it, otherwise you can spoil it.
2) Let’s open the router settings by typing in the browser the address http://192.168.1.1 (maybe 192.168.0.1) and enter the standard login – admin, password – admin.
3) In the interface that opens, click “System Tools” – “Firmware Upgrade” at the bottom, where then we click “Browse” and select the firmware file downloaded earlier and unpacked from the archive, then click “Upgrade” to start the update process.
Wait for the update to complete, usually 2-5 minutes. At the end, the router will reboot.
Done. After the firmware, the settings were reset to the factory settings.