You can do this in My Account under "My Agreements", by clicking on "Update payment card".
Select the BroBizz or PayByPlate agreement you wish to update your payment card for, and then click on "Payment card" or "Petrol card", depending on the payment method you wish to use for the selected agreement.
You can also update your payment card by using the BroBizz app. The app can be downloaded from Google Play and the App Store.
If you cannot remember your login information for My Account, you can also update your payment card without logging in, by using your BroBizz number and the postal code where your agreement was registered.