@subham_das, certbot used to be called letsencrypt more than one year ago. They are the same program. Both forms are still supported but we are trying to encourage people to refer to certbot, which is the new name. Depending on how people installed the program, the appropriate command for them to run might be certbot-auto or certbot. Wherever you see documentation referring to any of these forms, you should substitute the appropriate one for your system. The result of choosing the wrong one would just be a “command not found” error.
The recommended way to renew certificates is certbot renew, which ideally should be run automatically at least once per day, normally using cron.
certbot renew checks all of the certificates that you’ve obtained and tries to renew any that will expire in less than 30 days.
If you do want to renew a specific certificate manually, you can use certbot certonly --force-renew and specify all of the associated domain names with -d (e.g., certbot certonly --force-renew -d example.com -d www.example.com if both of those names are part of the certificate).
As an alternative to specifying all of the names, you can run certbot certificates to find out which certificates you have and what their names are, and then specify the certificate’s name with --cert-name (e.g., certbot certonly --force-renew --cert-name example.com).