Here's how to change your payment info right in the app. The steps are pretty similar if you are logged in and using Patreon on a desktop.
Here is Patreon's help article on payments https://support.patreon.com/hc/en-us/articles/203913799-Why-did-my-payment-decline-How-do-I-retry-my-payment-#targetText=If%20your%20credit%20card%20continues,future%20payments%20from%20Patreon*Membership.