Find the current cert location

sudo vi /etc/nginx/nginx.conf

Look for lines

ssl_certificate /etc/pki/nginx/cert.pem;

Go to Digicert and download .pem with All certs

Use WINSCP to copy this to /etc/pki/nginx/and change the config to look at the new PEM file : 

sudo vi /etc/nginx/nginx.conf

Restart Nginx

sudo service nginx restart