Upgrade (stylized upgrade.html) is a screen that appears when a unaccepted browser is used to access Google Santa Tracker.
After placing the address of the site, the normal loading screen will show up and load normally for a few seconds until the link changes (at the end, "upgrade.html" is added). Then, the player will be redirected to a screen showing an elf and a snowman trying to repair a laptop, the message "Oops! Santa Tracker isn't supported in this version of your browser. To access Santa Tracker, download a modern browser. For more information visit whatbrowser.org." and a red button saying "Go to the Village". If the respective button is tapped or clicked, the link will change again (with "?ignore_browser_check=true" and the end) and the game can be played, although with some issues.