To apply credits you would go to the Billing tab > Overview. From here, you will select the parent(s) that you wish to apply credits to by selecting the dot by their name(s). Then you will need to click the top drop down menu and select Apply Credits. To apply, click the check box under Apply and it will apply it to the latest outstanding invoice. You will do this for all parents in the list. Click Save & Close when you are done.

If you wish to apply a credit for one payer, you would go to their billing > click the down arrow in the payer's billing account > click the drop down menu > select Apply Credits. To apply, click the check box under Apply and it will apply it to the latest outstanding invoice. Click Save & Close when you are done.