Log into the Gaia webUI and go down to the Upgrade option under Maintenance.
Notice above the arrow on the left. That is how you get to where you do the upgrade. Upload the image (.tgz file) and its going to ask you if you are sure you want to do the upgrade (which I dont like). Answer yes.
Wait for the install to finish and it will reboot. Log back in to Gaia and go to Overview page and verify the upgraded image version. You are good to go.