I had to do this today for an Exchange 2003 server which was no longer permitting Outlook RPC over HTTP access. OWA still worked, but remote clients were stuck in a "Connecting" state. I found the following snippet at
MSExchange.org:
To renew a certificate, goto Control Panel > Administrative Tools > Internet Information Services (IIS) Manager. Find your "Default Website", right-click on it, and select Properties.
Under the "Directory Security" tab, click the "Server Certificate" button, then click "Next" on the wizard welcome screen and select "Renew the current certificate"
Once I did that, clients immediately reconnected.