Welcome to planDisney, Becky!
In order to upgrade your one-day, one-park
ticket to a Park Hopper ticket, you will need to stop by a ticket booth at the Disneyland Resort. Currently, there isn't a way to do this upgrade online or using the Disneyland app.
I definitely suggest upgrading your ticket to the Park Hopper option as it is so easy to hop back and forth between the parks. Also, it allows for your park touring to be really flexible. For instance, you can start your day riding iconic attractions like Pirates of the Caribbean and Matterhorn Bobsleds at Disneyland Park and then end your day with a dinner and a drink at the
Lamplight Lounge in Disney California Adventure Park.
If you already have a
theme park reservation made for your current ticket, you will keep that reservation and start there that day. Then, with your newly upgraded Park Hopper ticket, you can hop over to the other park starting at 1:00 p.m. You can even hop back and forth the rest of the day if you wish (subject to park capacity limitations).
I hope this helps! Have a wonderful time at the Happiest Place on Earth!
-Linda