How do I switch from a commercial SSL to Let's Encrypt?

I have the same problem

If you’ve already run through the Let’s Encrypt installation then we'll want to start looking at the web server configuration.

You can use the following command to find the loaded virtual hosts if you're using Apache
apachectl -S

and for Nginx:
nginx -T or ls /etc/nginx/sites-enabled

You'll want to ensure that the files mentioned in your virtual host is pointing to the location of where the Let’s Encrypt certificate and key are stored. Usually this is in the /etc/ssl/ directory, however, Let's Encrypt will store certificates in /etc/letsencrypt/live/ by default. You can copy these to the locations that are referenced in the virtual host or adjust the path in the vhost configuration file to point to /etc/letsencrypt/live/$yourcertfiles. Once you've done so, restart your web server.

I try adjust path in vhost configuration file point to /etc/letsencrypt/live/[mycertfiles.pem]

Problem solved.

Thank Bro