To exchange your ticket, go to your order via the "Manage my order" link you received in the confirmation e-mail sent, to get to your order.

If you can't find the e-mail, you can have it sent again here.

Then click on "Additional actions" and then on "Change tickets".

The system will then ask you which ticket(s) you wish to exchange. When you confirm the ticket you wish to exchange, on the next page you'll see the concert passes and tickets you wish to exchange.

If you exchange a Reflektor ticket for another Reflektor ticket, for example, you don't have to pay anything extra. If you want to change from a concert ticket to a 4-day pass, you only pay the difference.
 

Here's a link you can use to have this e-mail sent to you. 

This is just an order confirmation. Your tickets will arrive in early May.