You can switch to the Free plan at any time.
While on the dashboard, go to Account administration - Plan and Billing details and click on the text reading click here to change your pricing plan and choose the Free plan.
Free accounts are limited to 2 users and 2 Kanban boards only, with no file attachments. If you’re currently exceeding the limits, you first need to suspend additional users and delete excess Kanban boards before switching to the Free plan.