Click the profile icon in the top right of your portal. Select the 'Top Up Balance' button. Alternatively, you can also navigate to the Billing and Payments page, and select the Top Up tab.

Add the amount you want to top up your balance with, select a payment method and click 'Purchase'. You can also manage automatic top-ups from here.

If you are currently using the old user portal:

Go to Top up balance in your Billing and Payments section and a wizard will take you through the necessary steps where you can specify the amount that the balance should be topped up with. Add the item to the cart, proceed to checkout and make your payment.