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On the master :

kubeadm token create

On the node to join : (replace the token by the token gave by the master) replace 10.1.1.2 by your master kubernetes api IP

kubeadm join 10.1.1.2:6443 --token nohc43.abunuby45truc --discovery-token-ca-cert-hash sha256:456789fghj45678fghjk

# first upgrade the controller

kubectl get pods --all-namespaces -o wide
apt-get update
kubeadm upgrade plan
apt-get install kubeadm=1.12.6-00
kubeadm upgrade plan
kubectl -n kube-system get cm kubeadm-config -oyaml
kubeadm upgrade apply v1.12.6
kubectl -n kube-system get cm kubeadm-config -oyaml

cd /etc/kubernetes/manifest
edit kube-apiserver.yaml 
+add - --insecure-bind-address=127.0.0.1
+add - --insecure-port=8080

#Upgrade on nodes

apt-get update
kubeadm upgrade node config --kubelet-version v1.12.6
apt-get -y install kubelet=1.12.6-00 kubeadm=1.12.6-00
systemctl restart kubelet

#On master

kubectl drain s3-ny4-1 --ignore-daemonsets
kubectl get pods --all-namespaces -o wide
kubectl uncordon s3-ny4-1
kubectl get nodes