Upgrade k8s cluster

The official doc

https://v1-30.docs.kubernetes.io/docs/tasks/administer-cluster/kubeadm/kubeadm-upgrade/

Changing the k8s package repository

https://kubernetes.io/docs/tasks/administer-cluster/kubeadm/change-package-repository/

update repo gpg key

When you update k8s package from the repo, you may encounter the below problems

Err:4 https://prod-cdn.packages.k8s.io/repositories/isv:/kubernetes:/core:/stable:/v1.31/deb  InRelease
  The following signatures were invalid: EXPKEYSIG 234654DA9A296436 isv:kubernetes OBS Project <isv:kubernetes@build.opensuse.org>

W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://prod-cdn.packages.k8s.io/repositories/isv:/kubernetes:/core:/stable:/v1.31/deb  InRelease: The following signatures were invalid: EXPKEYSIG 234654DA9A296436 isv:kubernetes OBS Project <isv:kubernetes@build.opensuse.org>

W: Failed to fetch https://pkgs.k8s.io/core:/stable:/v1.31/deb/InRelease  The following signatures were invalid: EXPKEYSIG 234654DA9A296436 isv:kubernetes OBS Project <isv:kubernetes@build.opensuse.org>

W: Some index files failed to download. They have been ignored, or old ones used instead.

The problem is caused by the repo who has changed its pgp key. To fix it, you need to download the new gpg key.

sudo apt-get update
# apt-transport-https may be a dummy package; if so, you can skip that package
sudo apt-get install -y apt-transport-https ca-certificates curl gpg

curl -fsSL https://pkgs.k8s.io/core:/stable:/v1.31/deb/Release.key | sudo gpg --dearmor -o /etc/apt/keyrings/kubernetes-apt-keyring.gpg