Sometimes we make mistakes in purchasing the wrong app by purchasing the iPhone app instead of the iPad app and wish we could remedy the situation. Well normally Apple doesn't refund money for purchased apps, but there is a means to at least report a problem to Apple and possibly receive a refund if the reason is substantial.
If you have received a receipt for the purchase of the app via email, you can click on the Report a Problem link within the email to take you to your iTunes Store Account.
Alternately, if you do not have that receipt you can click on the iTunes Store in iTunes, click on the home icon to take you to the home page of the iTunes Store. Here you will click on the link 'Account', on the right side under Quick Links.