If resale tickets are available you will be able to purchase them from the event page, in the same place you would go to buy a regular ticket for that event.
Follow the purchase process by selecting your ticket type and quantity, enter attendee details, then process payment.
Ticket availability is dependent on when other fans decide to sell back their unwanted tickets. If tickets are available, you'll be able to select a quantity of tickets from a drop down. If there are no tickets currently available to purchase you will see Allocation Exhausted under the Quantity column.
*Resale is not available for all events