Navigate to your Business (This will depend on what your business manager account is called)
View 'Business Settings' by selecting the "Cog Wheel" icon at the bottom left of the screen.
βNavigate to Accounts > Ad Accounts > Select your ad account in the list
βBeside the 'Open in Ads Manager' button is a Down-Arrow (β¬) which you will Click > Select 'View Payment Methods'
βAt the top-right of the page, you should see a blue 'Pay Now' button. You can select this to keep your account up-to-date.
Note: After paying, if your campaigns were previously active, your ads will automatically resume.