We have enabled https 8443 port on both Author and Publish instances but while creating certificates we have mentioned 365 days now it got expired so we need to renew it. I need to automate the process by using Let's encrypt or Certbot but I am not sure how to implement it using those tools.
If you can live with the fact, that the connection between dispatcher and AEM is unencrypted, you can just use the standard tools to keep the webserver certificate uptodate (the one which is end-user facing).
OOTB AEM does not have an SSL "automate the renewal process". I think that if you manage to automate SSL auto-renewal with Let's Encrypt Project", then with a curl script suggested by Jaideep you can achieve the last step of upload new cert to AEM.