To return an account's bill unit to one of its own payment methods:
From the Actions menu, select Financial Setup, and then select Bill Unit Setup.
The Financial Setup screen appears.
Click the pencil icon in the bill unit to assign, or click New to create a bill unit to assign.
In the Payment method of Paying Account drop-down list, select an On File payment method or an option to create a new payment method.
For more information on adding a new payment method, see "Adding a Payment Method".
Click Apply.