1. First, download a firmware file. Make sure the firmware is correct for your router model. Also, make sure its in its original file extension.. You can't upload a .zip file.
2. With that in mind, go to http://192.168.1.1 and login into your router

3. Once you are logged in, go to the "Administration" tab.

4. After get there, click "Firmware Upgrade".

5. Lastly, click "Browse", find the firmware file and click Upgrade. Do not shut off your computer or your router. If the connection is lost, the firmware will fail.

If the upgrade was a success, it will say "Upgrades are complete". Good luck with your new firmware!