VMware has a guide to help you with the process of doing the certificate replacements, the document is called "Replacing Default vCenter 5.1 and ESXi Certificates".
If attempting to replace a certificate please do consult this guide and keep in mind that it is not going to be an easy task.
But wait VMware has taken their vCenter Certificate Automation Tool out of Beta and made it publicly available with the introduction of version 1.0
Details on the features you can find on the VMware Blog "VMware Support Insider".
With version 1.0 certificates of the following VMware products can be changed / update with the use of this tool:
- vCenter Single Sign On
- vCenter server
- vCenter Inventory Service
- vCenter Log Browser
- vSphere Web Client
- VMware Update Manager (VUM)
- vCenter Orchestrator (VCO)
Hopefully in the next releases they will add more products which can be managed by this tool, like the ESXi hypervisor itself, vCloud Director, Site Recovery Manager and vCenter operations.
For details on deploying and using the tool, please have look at the KB article VMware has published under kb2041600