From your dashboard, click "Manage" on the ticketed event you wish to edit. Then click on "Tickets" on the left-hand side menu.
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Choose the ticket whose quantity you wish to adjust and click the "edit" icon, indicated by the pencil.

To adjust ticket quantity, edit the number in the "Quantity" field. To adjust the number of tickets with each purchase, edit the "Tickets Per Package" field. Click the blue "Save" at the bottom of the page, and you're done!