To update your primary PayPal funding source, you will need to login into your PayPal account at paypal.com and follow these steps:
- Go to "Profile -> More Options"
- Next to "My preapproved payments", click "Update"
- You should see Facebook listed as one of your merchants
- Click "Set Available Funding Sources"
- Choose which funding source you'd like to use for payments to Facebook in the future
- Click "Save"