You can use the kubectl patch method to perform an "in-place" update of a Tanzu Kubernetes cluster. The kubectl patch method is an alternative to using the kubectl edit command to perform one of the supported cluster update operations.

About the Kubectl Patch Command

Restriction: Do not attempt to use kubectl patch as described in this topic to update a cluster specifiction so that it conforms with the TKGS v1alpha2 API. For this type of update you should use kubectl edit. See Updating a Tanzu Kubernetes Release After the Cluster Spec Is Converted to the TKGS v1alpha2 API.
The kubectl patch command performs an "in-place" update of a cluster. The purpose of the this command is to provide a method for upgrading Kubernetes versions, and is the approach documented here. For more information on the kubectl patch command, see Update API Objects in Place Using kubectl patch in the Kubernetes documentation.

The approach demonstrated here uses the UNIX shell command read to take input from the keyboard and assign it to a variable named $PATCH. The kubectl patch command invokes the Kubernetes API to update the cluster manifest. The --type=merge flag indicates that the data contains only those properties that are different from the existing manifest.

Upgrade the Kubernetes Version Using the Patch Method

The most common method to trigger a rolling update is by changing the Kubernetes distribution version for the cluster, using the .spec.distribution.version and .spec.distribution.fullVersion properties. You update the version hint and unset or null out the fullVersion to avoid a potential version mismatch during discovery.

$ read -r -d '' PATCH <<'EOF'
spec:
  distribution:
    fullVersion: null    # NOTE: Must set to null when updating just the version field
    version: v1.18.5
EOF

Apply the update using the kubectl patch command. You must include the quotes around the "$PATCH" variable to preserve newline characters in the cluster manifest. Replace the TKG-CLUSTER-NAME value with the actual name of your cluster.

kubectl patch --type=merge tanzukubernetescluster TKG-CLUSTER-NAME --patch "$PATCH"
Expected result: 
tanzukubernetescluster.run.tanzu.vmware.com/TKG-CLUSTER-NAME patched

Update by the Cluster Changing the VirtualMachineClass for the Nodes Using the Patch Method

Another way to trigger a rolling update of a Tanzu Kubernetes cluster is by changing the VirtualMachineClass for the node pools, that is, by changing the property .spec.topology.controlPlane.class or the property .spec.topology.workers.class.

read -r -d '' PATCH <<'EOF'
spec:
  topology:
    controlPlane:
      class: best-effort-xlarge
    workers:
      class: best-effort-xlarge
EOF

Apply the update using the kubectl patch command, replacing the variable with the cluster name.

kubectl patch --type=merge tanzukubernetescluster TKG-CLUSTER-NAME --patch "$PATCH"
Expected result: 
tanzukubernetescluster.run.tanzu.vmware.com/TKG-CLUSTER-NAME patched

Update the Cluster by Changing the StorageClass for the Nodes Using the Patch Method

Another way to trigger a rolling update of a Tanzu Kubernetes cluster is by changing the StorageClass for the node pools, that is, by changing the property .spec.topology.controlPlane.storageClass or the property .spec.topology.workers.storageClass.

$ read -r -d '' PATCH <<'EOF'
spec:
  topology:
    controlPlane:
      storageClass: gc-storage-profile
    workers:
      storageClass: gc-storage-profile
EOF
Apply the update using the kubectl patch command, replacing the variable with the cluster name. 
kubectl patch --type=merge tanzukubernetescluster TKG-CLUSTER-NAME --patch "$PATCH"
Expected result: 
tanzukubernetescluster.run.tanzu.vmware.com/TKG-CLUSTER-NAME patched