I have tried to download 2 rented movies and it says I don't have enough memory. How do I cancel one of them so I can download the other?

Does it show in the Downloads tab in the iTunes store app for pausing and deleting, or have you got other content on your iPad that you could remove so that you have enough free storage for it ? Rentals are a one-time only download from the store, if you stop it downloading and want to download it in the future you may be charged again