Tap Change Plan and choose the number of vehicles and the period.
More vehicles: the new plan applies right away. It’s billed the same day, and Apple refunds the time left on the old one.
Fewer vehicles: the new plan applies at renewal, and the Subscription screen shows it (“From …”). If you have more linked vehicles than it covers, the most recently linked vehicle then becomes “History Only”. To keep a different one, unlink the vehicle of your choice yourself before renewal.
