In the Finance section, the first section is Orders. Orders gives a list of each order transaction that has taken place for your event. It includes an order number (which allows you to click through and view the transaction details) as well as the contact name, date, and an action button that gives you various options.
When you click on the Order Number, you will see the registrant's Confirmation. This will give you their basic billing information and allow you to re-send their confirmation to them as well as begin the process of issuing a refund.
When you need to do a refund, you can click on the registrant's order number. You can do this from the Orders section or from the Attendee Registrations section. You can reimburse all or just a portion of the order, and you have the choice to cancel the registration or just refund the amount without canceling the registration. There is a Notes field here as well so that you (or a colleague) can always go back and see why a refund was made.