Here is how to replace the SSL certificates on VMware Update Manager running under vCenter 5.1.  I found the text from the following document, just reposting for future reference/ease of access: http://www.vmware.com/files/pdf/techpaper/vsp_51_vcserver_esxi_certificates.pdf

1. Back up the existing Update Manager certificates.
2. Copy the newly created certificate files (rui.crt, rui.key, and rui.pfx) to the Update Manager SSL directory on the system where Update Manager is installed.
   1. Typically, the directory is C:\Program Files\VMware\Infrastructure\Update Manager\SSL.
3. Stop the VMware vSphere Update Manager service.
4. Change to the Update Manager installation directory.
   1. Typically, the directory is C:\Program Files\VMware\Infrastructure\Update Manager\.
5. Run the file VMwareUpdateManagerUtility.exe.
   1. Note, this might take a couple of seconds to startup, there was a delay for me.
6. In the Options pane, click SSL Certificate.
7. In the Configuration pane, select Followed and verified the steps and click Apply.
   1. You should receive the following text when it is done: "Successfully applied the configuration."
8. After the operation completes, start the VMware vSphere Update Manager service.