Hi Dwong. Try this:

1. Go to Public Mobile self serve page
2. Login to Selfserve, click Payment tab.
3. Below "One Time Payment" and "Manage Auto Pay" options there is a very small link "Manage my card - Update or remove my Credit or Visa debit card"
4. Two options are available: "Replace this credit card" and "Remove this credit card"
5. Use "Replace this credit card" to update the CC info

Let us know if that works.

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