The actions can be made in the platform from the left menu, under Setup > Admin > FlexOrgs.
Initially, all AWS accounts, Azure subscriptions, and GCP projects that you have configured in the platform are allocated to the Top-Level Organization Unit (TLOU).
When an account is assigned to a Sub Organizational Unit (Sub-OU), its data will get processed after the nightly bill processing.
When an account is unassigned from a Sub-OU, then all data from that account should immediately no longer be visible within the Sub-OU, however, if the account remains connected in the tenant, then its historical and current data should still remain in the TLOU.
More information on creating and managing FlexOrgs can be found here: https://techdocs.broadcom.com/us/en/vmware-tanzu/cloudhealth/tanzu-cloudhealth/saas/tnz-cloudhealth/using-and-managing-tanzu-cloudhealth-working-with-flexorgs.html