First, go to the web configuration page. This is usually located at
http://192.168.1.1/
log in as the administrator, i'm not sure what the default password is
for the wgu624 is. Most routers the username/password default is
admin/admin. Hopefully you actually changed this during install

Then from the main menu, go to Maintenance, then Router Upgrade. Then
you should see a Browse button so you can upload the .bin file you
downloaded. It's really important that the router is not powered down
or disturbed in any way during the next few minutes while the new
firmware is installed.
On the same screen that you upload the new version of the firmware,
there is an option that the router can check automatically for new
versions. This is handy, but if everything is working fine, there
isn't any reason to upgrade.