Log in to your Linksys web interface at http://192.168.1.1 (default password: admin)

On the main page, click on Connectivity on the left menu.

This will open up a new page. On the right side, under Router Firmware, untick Automatic and under Manual click Choose File.

Browse to the firmware file you downloaded from the portal and click Start.

You will be asked to confirm that you wish to proceed. After the file is transferred, you will see the following screens confirming the status:

The router will now reboot and within a few minutes should be available for use.