You can upgrade or downgrade to any plan at any time by selecting the plan of your choice on your Billing page:
When you make any change to your subscription plan on your Billing page, the change will take place immediately, meaning that your available resources will automatically update to reflect your new plan. If this change occurs during your current subscription month, your payments will be prorated accordingly.
For example, if you downgrade to a lower plan mid subscription month, you will have a balance remaining on your account based on how much you have already paid, which will be incorporated into your future payments. If you upgrade to a higher plan mid subscription month, you will only pay the difference between the two plans, and only for the number of days remaining until your next payment.