Hey there Natasha,
Welcome to planDisney and thank you for your question!
Currently, to visit one of the parks of the Disneyland Resort, you need both valid
theme park tickets and a
theme park reservation for each day you plan to visit. When you purchase theme park tickets through the Disneyland Resort website, you also make your theme park reservation as part of the purchase process.
Your theme park ticket is usually valid for a year or two after the purchase date - I typically check the expiration date for my tickets in the confirmation email I receive after purchasing them.
While park reservations are subject to availability and can run out for certain days throughout the year, the park reservation system is pretty flexible if the dates of your trip change!
My preferred method of changing theme park reservations is through the
My Plans hub on the Disneyland Resort website. You can find the date(s) that you'd like to change and follow the process starting from this page.
If your park reservation dates have already passed, you might need to
call a Cast Member at the Guest Information line to get some more information on getting your park reservation reset.
Thanks again for your question, Natasha! I hope you have a great time visiting the Happiest Place on Earth.
Have a Magical Day,
Greg