Within Teamwork Projects, you can downgrade your subscription at any time. 

Just go to the site settings menu in the top right of your screen and choose the 'Subscription' area:

Note: Only site administrators can access this area.

In the Subscription area, click the green 'Update Subscription' button below your current plan summary:

In the Update Subscription modal, click the 'full checkout' link to view your plan options.

To downgrade to the free-forever plan, just click 'Select' on the Free plan.

You can enter a reason for cancelling the current subscription, along with any additional comments. When you have cancelled the existing subscription, your account will be moved to the free-forever plan. You can resubscribe to a paid plan at any time, via your Subscription settings.

Note: In the event your paid subscription expires, your account will automatically downgrade to free-forever in trial mode, and you will see the purple banner below suggesting to 'Choose a Plan.'

Once you click this button, you will be able to select the Free Forever account as your new subscription plan, which will then remove the banner:

For more information, see: Upgrading your Teamwork Projects plan