Given an existing Kubernetes cluster running version 1.20.1，
upgrade all of Kubernetes control plane and node components
on the master node only to version 1.20.2

You are also expected to upgrade kubelet and kubectl 
on the master node。

Be sure to drain the master node
before upgrading it and uncordon it after the upgrade.
Do not upgrade the worker nodes,etcd,the container manager,
the CNI plugin,the DNS service or any other addons.

k get node

apt update

apt-cache madison kubeadm

apt-get update && apt-get install -y --allow-change-held-packages kubeadm=1.20.2-00

kubeadm version

kubeadm upgrade plan

kubeadm upgrade apply v1.20.2

k drain master-1 --ignore-daemonsets

apt-get update && apt-get install -y --allow-change-held-packages kubelet=1.20.2-00 kubectl=1.20.2-00

systemctl daemon-reload

systemctl restart kubelet

k uncordon master-1

k get node