Exchange certificate expired or not valid

We've got an Exchange 2007 server running on Windows Server 2008 in our small domain for internal use only, installed one year ago.

Today the outlook clients started to show a security warning: "the certificate is expired or not yet valid".

As I can see from the details, the certificate was valid from installation for one year only. It is an automatically generated certificate, there is no need for a official, since this server is only used internal.

Is there an easy way to extend the certificate, or replace it without much work?

Thanks,
Sam