To see the amount due on your next billing date, start by navigating to your billing portal by clicking the user icon in the upper right corner of your screen, then selecting Billing from the dropdown menu.

Alternatively, this may also be accessed through the sidebar:

From the Billing page, your next renewal total can be seen on the right hand side, in the Subscription Summary. This shows a breakdown of the upcoming charge, and will display any credits or coupons applied as well.
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From the Billing screen, click Update in the top right corner of the Plan block.

Further down the page you can increase your addon count for campaigns, rank tracker, and site auditor if applicable. When increased or decreased the renewal summary on the right will automatically update.