Refunds for Atlassian Cloud are only available within the first paid month after the trial period for monthly subscriptions and within 30 days of payment for annual subscriptions. After these periods Atlassian cannot offer refunds.
To cancel your trial/subscription for Atlassian Cloud, follow the instructions below:
Log in to your My.Atlassian account. You'll arrive on the main Licenses page. Atlassian Cloud will be listed with the unique URL.
Click the > next to your Atlassian Cloud subscription.
Click the Cancel Subscription or Cancel Evaluation link to request the cancellation of your account.
The Atlassian Cloud subscription will be terminated at the end of the current billing cycle and your credit card will not be charged again. No credit is offered for canceling an account prior to the end of a billing cycle.
To request a refund during the first month of paid service (following a trial) or within 30 days of payment for an annual subscription, contact Atlassian Customer Advocate Team.
Source: Atlassian Purchasing & Licensing FAQ