How do I renew a Web Server certificate in Windows Server 2008?

It is not possible to renew an expired certificate. You will have to issue a new certificate.

Reference: http://technet.microsoft.com/en-us/library/dd378790(WS.10).aspx

You cannot renew a certificate that has already expired. If you try to renew a certificate that has expired, the certification authority (CA) will reject the request, and you will see an error message similar to "Error Verifying Request Signature or Signing Certificate. A required certificate is not within its validity period when verifying against the current system clock or the timestamp in the signed file." This message will also be displayed in the Failed Requests node of the issuing CA. If your certificate has already expired, you must request a new certificate instead of renewing the existing certificate.

You can renew from IIS manager under administration tools. Start IIS manager and then expand your node. Right click web sites and then choose properties. Click Directory security and then choose secure certification