Tickets that are live on the event page can be edited and you can add extra ticket types.
Points of caution:
Once you have created a ticket, the ticket name as well as the ticket price cannot be changed. This is to avoid confusion with people that have already ordered tickets. If you want to change the details of a ticket, you need to delete it and then recreate the ticket.
On the "Ticket detail setting" page, you will see the following screen. Here, you can change the sales period of the ticket and change settings such as the maximum number of tickets that 1 person is permitted to order etc.
Important note for ticket limit settings
If you are selling multiple tickets types for a single event, set the minimum ticket limit to "0" (the default is "0"). If you set it to 1 or more, even when the user is purchasing a different ticket, they will also be asked to purchase the ticket that you have set to "1" or more.