When we check kubernetes it is showing below error.

Unable to connect to the server: x509: certificate has expired or is not yet valid

Checked the certificates and they are indeed expired:

# find /etc/kubernetes/pki/ -type f -name "*.crt" -print|egrep -v 'ca.crt$'|xargs -L 1 -t -i bash -c 'openssl x509 -noout -text -in {}|grep After'

bash -c openssl x509 -noout -text -in /###crt|grep After
Not After : Jan 13 18:37:22 2022 GMT

Followed the belw steps to renew the certificates in Kb8 ( if you are on K8s 1.17.9 or above):

1) make sure you take a copy of all the crt files as well as these:

/etc/kubernetes/admin.conf
~/.kube/config/admin.conf

2) run these commands

kubeadm alpha certs check-expiration
kubeadm alpha certs renew all

3) copy /etc/kubernetes/admin.conf to your ~/.kube/config