Found this on uss website, similar arrangement of various theme parks~
Upgrade your Admission Ticket
Want to spend another day at the Park or have unlimited access to the park for the entire year?
Simply pay the difference and upgrade your 1-Day Ticket to a 2-Day Ticket or to either of the 2 Annual Passes.
Proceed to our Guest Services Window located at the main entrance to upgrade your ticket prior to entering the Park if the following scenarios apply:
> If you already have your unused, valid ticket in hand.
> If you have purchased your ticket online and it is before expiration date.
If you would like to upgrade your one-day ticket during your visit, please visit our Guest Services located along Hollywood Boulevard™ prior to exiting the Park.