Hey, I discovered that there is a handy certbot cli command (works on ubuntu 16.04)
simply type:

certbot

Gives you something like that:

Saving debug log to /var/log/letsencrypt/letsencrypt.log
Plugins selected: Authenticator nginx, Installer nginx

Which names would you like to activate HTTPS for?
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
1: [editted]
2: [editted]
3: [editted]
4: [editted]
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Select the appropriate numbers separated by commas and/or spaces, or leave input
blank to select all options shown (Enter 'c' to cancel):

Guys I can’t access my subdomain on http, its shows on https. Here are my nginx config.

listen 80 default_server;

Hi !
If you have create your vhost to your subdomain, you have just to do this command :

Install Certbot :

sudo git clone https://github.com/letsencrypt/letsencrypt /opt/letsencrypt
./letsencrypt-auto certonly --standalone

Certificate your subdomain :

sudo ./certbot-auto certonly --standalone -d sub.domain.com

It works fine for me. Don’t forget to restart nginx :

service nginx reload