How do I undo a mistake made in purchase?

Good luck getting a refund. This was not possible for me.

I purchased a 4 movie set using an iPad and iTunes.

I thought the set had both the theatrical and special editions of the movies.

Not correct. They were only the special editions.

I did not download the films.

Two days after purchase, I went to the emailed receipt, selected "Report a Problem", selected "Meant to purchase a different item" and wrote that I would repurchase the films individually (which have both versions) which would mean more money for Apple.

The response? "This purchase is not eligible for a refund."

- So, the Apple return policy is arbitrary. Sometimes there's a refund, other times no refund.

All purchases are considered final, but you can try the 'report a problem' link to contact iTunes Support and see if they will refund or credit you : http://reportaproblem.apple.com

If the 'report a problem' link doesn't work then you can try contacting iTunes Support via this page : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption

What do I have to do to delete or reverse the purchase of an app that was mistakenly downloaded?

All purchases are consider final, but you could try the 'report a problem' site to contact iTunes Support (try reading my above reply), or the 'contact us' link at the bottom right of every page on here

I want to cancel a p

Mistake pu

Nothing I can do, you will have to try contacting iTunes Support and see if they will refund you (purchases are considered final) : http://reportaproblem.apple.com