Enable Operating System Manager
Starting with KKP 2.21, OSM will be enabled by default for all the new user clusters. This can be configured using the dashboard or CLI.
Via UI
Enable OSM
Create a new cluster from the dashboard and toggle Operating System Manager feature on.

Enable OSM during cluster creation
OSM cannot be disabled after cluster creation.
Selecting OperatingSystemProfile

Select OperatingSystemProfile
Via CLI
On cluster creation, set the following values in Cluster
resource:
apiVersion: kubermatic.k8c.io/v1
kind: Cluster
metadata:
name: crh4xbxz5f
spec:
...
enableOperatingSystemManager: true
...
Custom OperatingSystemProfiles
To consume custom OperatingSystemProfiles. Users can create their custom OSPs in the seed namespace(kubermatic). They will be automatically synced to all the user-cluster namespaces.
Migrating existing clusters
For migrating existing clusters, user can enable OSM using either the CLI or UI. That would enable OSM on the user cluster level. Although the machines will not be rotated automatically. To perform this rotation for existing MachineDeployments please follow the guide at Rolling Restart MachineDeploments.