Get the version of the API server:

kubectl version --short

Release the hold on versions of kubeadm and kubelet:

sudo apt-mark unhold kubeadm kubelet

Install version 1.18.5 of kubeadm:

sudo apt install -y kubeadm=1.18.5-00

Hold the version of kubeadm at 1.18.5:

sudo apt-mark hold kubeadm

Verify the version of kubeadm:

kubeadm version

Plan the upgrade of all the controller components:

sudo kubeadm upgrade plan

Upgrade the controller components:

sudo kubeadm upgrade apply v1.18.5

Release the hold on the version of kubectl:

sudo apt-mark unhold kubectl

Upgrade kubectl:

sudo apt install -y kubectl=1.18.5-00

Hold the version of kubectl at 1.18.5:

sudo apt-mark hold kubectl

Upgrade the version of kubelet:

sudo apt install -y kubelet=1.18.5-00

Hold the version of kubelet at 1.18.5:

sudo apt-mark hold kubelet