how to renew letsencrypt certificate outside plesk ?
Hi @alhwary and welcome to the LE community forum 
Only if you must do this outside plesk, [not sure why plesk can't fulfill your needs - but that's another topic] It depends on which ACME client is being used and how much access you have to the root system.
More than likely it is using certbot and you probably have root level access [or you wouldn't be here].
If so, then start with [show us]:
certbot certificates
to see what you already have and then we can go from there.