Did you try looking into godot docs for reference to the specific things that you need?

https://docs.godotengine.org/en/stable/index.html

Upgrade is a theme for a game not a component of the engine.

So the contest is using the Godot engine to design a game based around the theme of 'upgrade', which could be a player who levels up and gets stronger or a clicker type game where you're upgrading the rate at which you earn resources for example.