How to cancel pending purchase in Appstore?

If it is pending the purchase has already been made, you can't cancel it.

You can request a refund by following the steps in this guide:

Request a refund for apps or content that you bought from Apple - Apple Support

You are not guaranteed a refund tough.

This is an automated process, if you wish to explain your situation in-depth to a person you can:

Contact Apple for support and service - Apple Support

Also, see this:

How long refunds from the App Store or other Apple services take - Apple Support

Good luck!