SSL Commercial Certificates
If you need to install a commercial certificate, there are seperate instructions for Zimbra 4.x and Zimbra 5.x:
If you have a single 5.x server (non multi-server), you can just use the Install Certificate function in the Admin console to generate a request and install a commercial certificate. Otherwise you'll need to use the ZCS Certificate CLI tool zmcertmgr.
If you have a self signed (not commercial) certificate you may also find Recreating a Self-Signed SSL Certificate helpful.