Hey there Barbara,
Welcome to planDisney from a fellow Arizonan! I love getting the chance to answer questions from a fellow resident of the Grand Canyon State.
Currently, when you 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. The tickets are a one-time purchase and then they are valid through their expiration date, which you can find on the purchase confirmation email that you receive.
The theme park reservations are made for a specific date and park and are what actually allow you to enter a park on a given date. It's your theme park reservation that we'll need to change for your trip next month so you can visit Disneyland Park instead of Disney California Adventure Park!
You typically make your theme park reservation as part of the theme park ticket purchase process, but you're able to modify it at any point after, but just note that theme park reservations are subject to availability and can run out especially as you get closer to the date of your visit.
To modify your theme park reservation, I'd recommend visiting the
My Plans hub on the Disneyland Resort website. This is a really helpful and convenient page where you can make and manage all of the different kinds of reservations for upcoming trips. To change the park for one of the days, you can click the modify link next to that day and follow the step-by-step process.
Thanks again for your question, Barbara! I hope you enjoy your upcoming trip to the Happiest Place on Earth.
Have a Magical Day,
Greg