When you log in for the first time, a pop-up about Earn will appear automatically. It explains what Earn is and how it differs from Vault. At the end, you can choose to move your assets to Earn or skip this for now.
Dismissed the pop-up? Navigate to the Earn section in the menu (web) or on the home screen (app). Then click 'Introducing Earn' to get started with moving your funds to Earn and follow the steps.