On PC web browser

1. When you log in to your Public Mobile account, go to Payments.
   
2. On the right hand side, click on Manage Subscription.
   
3. Then click on Manage Payment Method and remember, change and edit everything even if it’s already auto filled.
   

On mobile app

1. When your app opens you’ll be in the My Account tab. You will see a plus button at the bottom left above the My Account tab. Click on it.
   
2. Once you’ve clicked on it, go to Payments
   
3. The next screen you’ll want to click on Manage Subscription.
   
4. The next screen will give you the option to change your credit card information when you click on Manage Payment Method.
   
5. There you will be able to enter a new credit card and expiry date. Click on Save and you're good to go.
   

Source