First, register your sonar with Humminbird. Only takes a few minutes. Once you do then your account for your model you register with will have any downloads for your sonar. Just download them to your desktop. Save the file on an SD card. It can be the same card you use to store your way points and recordings on. Then go to your sonar and power on. Set the unit to factory defaults. Then put the card in the slot and the unit will find the update and ask you if you want to update. Say yes and do not unplug until done. Once done it will reboot and you are updated. There are instructions on the Humminbird site where you download the update as well. Took me all of 5 minutes.
http://www.humminbird.com/Support/MyHumminbird/