You can downgrade to a basic free plan whenever you want. Downgrading to a Basic plan constitutes a subscription plan cancellation.
To do so, click Account from the dropdown menu under your name at the top of the page. Then click Subscription and Usage> Downgrade Plan.
This will bring up the pricing table. Select "downgrade" in the column of the basic plan.
Cancellation of the paid subscription will take effect at the end of the current billing period. Up to the end of the current billing period, access to the quotas and features of the paid plan will remain in place. At the renewal date you will be switched to a Basic user plan. You will be subject to the quotas and features of the reduced plan.