Some events allow upgrade to a higher registration type from the ticket purchase website itself. In this case, access your personal area from the web home page:

Login with your username and password and access My Purchases. From there, press the Add Product / Upgrade button:

Select the new ticket type  and continue with the change following the steps that will be indicated.

If the Add Product / Upgrade button is not active, it means that the show does not allow you to change the type of ticket.