The car may already have been reserved by someone else after you. In that case, you cannot extend your reservation, and you should return the car by the end of the period you originally booked for.
If you cannot be back on time as a result of unforeseen circumstances (car breakdown or an accident), you should phone MyWheels. The phone number is in the app in your current trip.
Is it too late to extend? Then you will have to make a new reservation. If this is problematic, you should phone MyWheels.