To change your plan first log into your account.
Click on the "Manage Account" tab then on the left hand side click "Change Plan."
Note: If you are selecting a plan for the first time, you must make a payment to activate the plan.
If you are changing plans, your new plan will take effect immediately for your maximum number of contacts and number of included keywords. However, your message credit balance will not be increased or decreased until the next billing cycle. If you need more text message credits before then, please purchase additional credits now. Also, you will not be billed for this plan change until the next billing cycle.
You can change your monthly plan only once in every billing cycle.
You can't downgrade to a plan with a lower number of Max Contact Size or Max Mobile Keywords than what you are using in your account currently.