I will describe the steps in steps:
1) Download the new firmware from the official site
Firmware is under the version of the routers V1/V2/V3 etc. You need to download the firmware for the version of the router that is written on the label under it, otherwise you can spoil it.
2) Connect to the router in any of the LAN ports, open the settings by typing in the browser the address http://192.168.0.1 (maybe 192.168.1.1) and enter the standard login – admin, the password – admin.
3) In the opened interface, click on the bottom of the “System Tools” – “Firmware Upgrade”, then click “Browse” and select the firmware file downloaded earlier and unpacked from the archive, then click “Upgrade” to start the upgrade process.
Wait until the update is complete, usually 2-5 minutes. At the end, the router will reboot.