You can increase or decrease the number of tickets left to sell for each ticket type. To do this select the event you wish to manage in the My Events screen and navigate to Event Details > Edit Tickets in the Dashboard menu.
Find the tickets you wish to edit and select the 'Change quantity' button. You can now select the number of tickets you have left to sell, you can increase or decrease the number.
Note that if you set all your tickets types to have no tickets left to sell then the event will go off sale.