Swapping tickets (GA and Reserved Seating) is done by admin users, not by customers. For information on self-service ticket exchanges, processed by customers themselves, please see our Exchange help article.
The ticket swap permission is set by client support. If you do not see this feature in your account, please contact support for information.
For info on swapping reserved seats, please refer to Swapping Reserved Seating Tickets.
How To
From the order details page, select the Tickets drop-down menu and choose Swap Items.

Click to open the dropdown and find the event you want to swap tickets to. Select the event, and if it's part of a series, also select the date and time.

For each ticket that you want to swap, select the price level that you are swapping to.

Choose whether to notify the customer of the swap by email. Then select Save & Continue.
You may see a warning message if the ticket has a different cost or if the ticket has already been scanned. You can choose to cancel the swap or to continue.
If you choose to continue, the swap will complete, and you will be redirected back to the order details page, where you can see that your swap has finalised.