If you're running low on credits, it's typically most cost-effective to upgrade your plan. You're able to upgrade and downgrade as often as needed.
However, if you prefer to purchase more credits separate from your plan, just head to Settings > Plan > Buy Credits. These credits (called rollover credits) do have a higher per-credit cost compared to monthly plans, because if they're not used, they will roll over to the next month. They are also the last credits to be used each cycle, and they never expire.
Note: Only an account owner or admin has access to the Billing Section and Plan page.