I accidentally purchased an app yesterday that is pending. How can I cancel and get a refund

Too late. Once it's been processed, it must be allowed to complete and charge you correctly, there is no way to cancel it at that point.

Once that happens you can request a refund. Link-> Request a refund for apps or content that you bought from Apple - Apple Support

MpiWarbonnet wrote:

Please tell me how to delete an app I accidentally purchased

Deleting is done the same way for all Apps purchased and free: click here-> Delete apps on your iPhone, iPad, and iPod touch - Apple Support

Follow the steps in the "Request a refund" link provided in the very thread you replied to if you need a refund for it.

If the App has a recurring subscription, follow this steps here to cancel it: link-> How to cancel a subscription from Apple - Apple Support

Please tell me how to delete an app I accidentally purchased