Log in to the router web interface at http://192.168.8.1
Click the Advanced Settings at the top right and login as normal.
Under System at the top, choose Backup/Flash Firmware.
Under Flash New Firmware Image un-tick (disable) the Keep Settings box, and then click Choose File to browse to the firmware file provided.
Click Flash Image and then Proceed on the following page.
The router will now upgrade and should be ready within 5 minutes.