You do not need to take any further action to cancel your subscription. At the end of your current subscription period, you will receive a request to renew. If you do not renew, your account will automatically be converted to a free subscription at the end of that period.
You will still have access to your existing invoices. However, you will no longer be able to access quotations and other modules. Make sure to download these files in time if you may need to consult them later.