Install free SSL certificate on nginx & apache, lets-encrypt provides free ssl certificate for any domain and subdomain.  go to the letsencrypt  Then click on  Certbot, and after that choose your software and system. Flow the default instruction.

Or you can try it.

Open the terminal and type

sudo apt-get update

sudo snap install core; sudo snap refresh core

sudo snap install --classic certbot

sudo ln -s /snap/bin/certbot /usr/bin/certbot

sudo certbot --nginx  

You can add multiple domains by space or coma

Note For apache

sudo certbot --nginx  

Update SSL Certificate

sudo certbot renew --dry-run