A plan can be changed at any time from the service's page. Upgrades take effect straight away; the price is adjusted for the time left in the current period.
Change the plan
Open the service under My Products & Services and choose Upgrade/Downgrade. Pick the new plan and, if you like, a new billing cycle, and confirm.
How the price is worked out
For an upgrade, an invoice is issued for the difference between the plans for the remainder of the current period; once it is paid the upgrade is applied. For a downgrade, the difference is credited to your account and applied to your next invoice.
Changing only the cycle
Choose the same plan with a different cycle. The new cycle starts at the next renewal, so nothing is charged twice.