I can never remember how to update SSL certificates on Plesk 11 from the command line.

To install a certificate (as ‘root’) use the following command:

/usr/local/psa/bin/ipmanage -u 180.235.134.238 -ssl_certificate NAME_OF_CERTIFICATE 

The names of the certificates can be obtained with:

/usr/local/psa/bin/ipmanage -s

From the web UI, use the following steps for installing SSL certificates:

1. Add your SSL certificate to the server using:
   Tools & Settings >> SSL Certificates
2. Follow the steps below:
   Tools & Resources >> IP Addresses
   2.1. Click the the IP: ???.???.???.???
   2.2. Change SSL certificate name from the drop down list (Select recently you have added).
   2.3. Click Ok button. 

Hope that helps someone.