How can I switch to the Free plan?

You can downgrade your account to the Free plan at any time.

While on the dashboard, go to Account administration → Plan and Billing details, click on the Change your pricing plan » link, and choose the Free plan.

Note, that free accounts are limited to two users and two Kanban boards only, with no file attachments support.
If you’re currently exceeding these limits, you will need to suspend excess users and delete excess boards before switching to the Free plan.