There are a few reasons you might not be able to update your mileage in the app.
You're close to renewal. If you're coming up to the end of your policy, you don't need to update your mileage. When you try to in the app, you'll get a message telling you this.
You're selecting a mileage beyond the limit of your policy. All policies have an acceptable mileage range, so if you can't change your mileage in the app, start a chat to discuss this with us.
The app has run into to an issue. You'll usually see a message telling you if there's been a problem, so just start a chat for help.