Q: cancel movie

All sales are final the moment you click buy/rent

As RG said, you probably cannot get a refund since the terms of sale for the iTunes Store state that all sales are final. You can contact the iTunes Store, explain the reason for your request, and ask, though:

http://www.apple.com/support/itunes/contact.html

It's possible they'll make an exception for you, though if the problem was on your end with a slow Internet provider, it's not likely.

Good luck.

When you buy an item from the iTunes Store, the amount of time it takes to download your purchase will vary according to the file's size and your Internet connection speed.

Download times will vary

You may want to stick with SD intead of High Definition next time.