Update certificate using the following instructions (a bit modified for update)

1. Add your subdomain
(I don't need this step usually).  You can do this as a normal subdomain or as a standalone domain.

2. Add your certificate to Plesk
Use the Plesk control panel to add your certificate. Where the shortcoming comes into play is that Plesk will continue to use the default certificate.  This is done from the domain account.

3. Find your certificate
Head over to /usr/local/psa/var/certificates
Take note of the file name cert-xxxxxx that has today’s creation date. There may be two if you installed a CA certificate. You will need to determine which file is the normal certificate and which one is the CA one for the step 5.  CA certificate is typically the larger one.

4.Copy the include file
This step is only needed the first time.
Go to /var/www/vhosts/sub.domain.com/conf and open the httpd.include file
Find where it has
ServerName sub.domain.com:443
Copy the entire VirtualHost block for JUST the SSL portion. (from <VirtualHost xx.xx.xx.xx:443> to </VirtualHost>)

5. Make your include file
Head over to /etc/httpd/conf.d and make a file. Call it something like z0-subdomain.conf. The file just needs to load after ssl.conf but before zz010_psa_httpd.conf. Paste what you copied from step 4 into the file and then set the certificate file names from step 3 into these following places.
SSLCertificateFile /usr/local/psa/var/certificates/cert-xxxxxx
If you have a CA certificate, you must also add this line just below:
SSLCACertificateFile /usr/local/psa/var/certificates/cert-xxxxxx

6. Restart Apache
service httpd restart
Most of these instructions were taking from this page, but they left the CA certificate step. This is very important since certain browsers do not trust all intermediate CA’s. (mostly Apple products and some cell phones)