The Account Owner of your account can upgrade or downgrade plans at any time. To change your subscription go to account admin > your subscription. Select the new plan you require from the drop down menu, click change plan and there you go you've changed your subscription.
If you are looking to downgrade and the plan you wish to downgrade to isn't showing in the select plan drop down menu, this could be because you need to delete some of the excess users before you can. You'll find more info here.
Assuming you pay for your subscription via bank card or direct debit, the charge will be automatically prorated to reflect your change of subscription.
If you're upgrading your subscription, a payment will be taken immediately to cover the remainder of the current billing period, and the first regular subscription payment of the new plan will be taken on your next monthly renewal date (usually the same day of the month as your Subscription Start Date).
If you're downgrading your subscription, any credit will be offset against the subscription payment due for the new plan on your next monthly renewal date.