Go to accounts.intuit.com. You can do this from within PTO by clicking on the gear icon for Settings. On the popup, click on Manage Your Account.
When you're in your account, under the click on Products & billing. Under your product, you'd see a link next to Payment and billing info for Manage payment method. That's where you change and add your billing methods.
You have clicked a link to a site outside of the Intuit Accountants Community. By clicking "Continue", you will leave the community and be taken to that site instead.