Step 1: Click here to open up your business manager by going to https://business.facebook.com.

Step 2: Once you are in your business manager, click on your ad account.

Step 3: Click on the 9 dots under the "house" icon on the right of the ad account to open the modal. Once the modal opens, click on "billing".

Step 4: Click the "payment settings" button on the right side of the page.

Step 5: Click "pay now" to pay the balance due.