This is on sales page for QBO in FAQ about mileage tracker feature. "As your business grows, QuickBooks Online may be the right option for you. The good news is you can automatically transfer your current mileage from QuickBooks Self-Employed to QuickBooks Online. After you sign in and switch plans you can select an option to bring your data over. You’ll find your miles waiting for you in your new account."
Where is this option exactly- in the app or online?