Canceling a plan is possible in the Settings module under the Billing tab. In the plan section, select Cancel plan (in the action menu marked with the three dots icon).
Cancellation of the plan will be done at the end of the billing period. Until then, users can use the application normally and the Organisation admin can revoke the cancellation decision.
Cancelling a plan will not remove the organisation, but users will not be able to use the application until the plan is selected again.
Check also
How do you choose a plan?
How long is the reference period?
Can I change the plan while it is in progress?