How can i cancel the mileiq i purchased last night?

Purchases are considered final but you could try the 'report a problem' page to contact iTunes Support and see if they will refund you as a one-off : http://reportaproblem.apple.com

Or you can try contacting iTunes Support via : https://support.apple.com/contact

And if it's a subscription : View, change, or cancel your subscriptions - Apple Support