You can change your monthly plan anytime!
Just go to the Plan & Billing page -> click on Upgrade options (or click here) and choose a plan. You can downgrade or upgrade whichever plan you'd like.
If you upgrade, changes take effect immediately and during the next billing cycle, you will be billed for the amount of the new plan + for the period you were on a new plan in the given month - how much time you had left on the previous plan.
In case of downgrading, changes will take effect during the next billing cycle.