You can do that at any time on the Billing page - you'll see the button at the bottom of the page:
After you cancel the subscription you'll still have a full access to Bïrch until the end of the billing period that you have paid for. Ad spend from ad accounts linked to Workspaces will keep being counted toward the total ad spend. If you'd like to stop ad spend from being counted, you can unlink ad accounts or disconnect the profile. Note: both of these actions - unlinking the ad account or disconnecting the profile - will cause rules to turn into drafts and Launcher templates and reports to disappear.