Active paid subscriptions cannot be renewed earlier than the date they are set to auto-renew on. However, you can stack plans upon each other. For this you would need to cancel your subscription from your 'My Account' page, and then purchase a new subscription. Cancelling a subscription still lets you have access to it until it expires. We don't renewing early though, as this can cause a mismatch between the plan expiry and the rebill date. We recommend cancelling your plan, waiting until it expires and then purchasing the new plan.
For example, let's say you have a monthly paid plan that renews on the 5th of every month. If you cancel your subscription on the 1st of the month and purchase a new monthly subscription right away, you will have the remaining 4 days from your cancelled subscription added to the new monthly subscription you just purchased. However, this will not move your billing date forward by 4 days, it will be the same day of the month as when you made the payment. So your plan expiry date will always be 4 days ahead of the re-bill date. This is why we advise you to wait until your plan expires, that way both the expiry and the re-bill dates are matching.