To that, Android Police provided a simple tip to fix the issue.
According to the tech publication, users who somehow missed the upgrade should open their Android Auto app on their phones and proceed to settings. After that, they can find a toggle option that says, “Try the new Android Auto.” Turn it on to complete the transition.
However, your mileage may vary. A couple of users in the AP’s comment section pointed out that not all users had received the menu option even in mid-August. For now, those without the toggle will have to exercise their patience with Google.
Source: Android Police