When you unlock the Platinum level, you'll be prompted to set your two favorite locations. Choose the two places you ride between most. Once your route is set, you should be able to see it listed under 'Favorite route boost' in the Uber Rewards menu of the app. If you don't see your route, uninstall and reinstall your app to ensure it's up to date.
If you don't activate this benefit immediately, you can set your locations later. Just look for 'Favorite route boost' under the Uber Rewards menu in your app.
Your chosen route can be changed once every 30 days by contacting support.