You can upgrade your subscription in the 'Billing' section.
Account Profile > Billing
You can upgrade your subscription at any time, you have two ways to do this.
You can change your plan for the next billing period, meaning you will be charged for the new amount on the first day of the next billing cycle and will not have access to the upgraded features until that time.
If you choose to change your plan for the current billing period; your features will be upgraded immediately and you will pay the prorated amount for the remainder of the current month.