Upgrading your license plan can be done by logging in to your account and then opening the Organization Settings

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Once you're in the Organization Settings, go to Plan & Billing tab
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Here you'll see your Total Contacts, Responses, SMS, and Billing details.

You can change your license plan by clicking CHANGE PLAN
‍(note that only the Owner of the organisation can change the license plan)
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Terminating your license can be done by changing the license plan to Free.