HOW TO transfer my applecare to my replacement macbook air

You cannot transfer AppleCare from one device to another but you can cancel it and, if your device is eligible, buy a new plan. See instructions in: Cancel or change the payment method for your AppleCare plan - Apple Support

The only exception would be if your replacement device was from Apple because your old one was replaced free with Applecare because it could not be fixed. In that case the remaining coverage would automatically be registered to your replacement device.