When you add a vehicle to the FIXD app, it will automatically add the manufacturer specific maintenance schedule, so you can document your service intervals. Under this same timeline feature, the app displays your mileage. It is important for the mileage to be correct so that the app can correctly prompt you about upcoming service intervals. To change your mileage, click the circular arrows on the top right corner of the timeline page. There, it will prompt you to update your mileage.
You can also update your mileage by going to Tire Treads under Wear Items. Click on one of the tire icons, and it will prompt you to enter the updated mileage.