Head to the plans page.
If you originally subscribed via Epoch, you can upgrade to a higher tier anytime by clicking "Upgrade" on the Plans page. You'll only be charged the pro-rated difference, giving you immediate access to the higher-tier features.
However, if you've previously canceled your subscription, upgrading won't be possible, as Epoch does not support this functionality.
Please note: the number of credits you receive will be proportional to the amount you're charged.
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If you subscribed via CCBill as your payment method, you will need to cancel your current subscription and purchase a new one to upgrade.