Dan’s link 
is correct. Blizzard Support - Contact Support The refund system is two parts. The first part is automated and granted if you meet the basic refund policy terms Blizzard Refund Policy - Legal – Blizzard Entertainment If the automated system does not grant it, you can put in a ticket to have a GM review it manually and determine if it qualifies.
Anything pre-order generally qualifies for refund unless the player uses something of value on the account, that was part of the pre-order and can’t be removed/locked.