How can I renew my expired ClickOnce certificate?

Here's the definitive MSDN article on certificate expiration, which also includes a link to an update on RenewCert. http://msdn.microsoft.com/en-us/library/ff369721.aspx This covers all cases.

If you are targeting .NET 3.5, using automatic updates, and don't have a VSTO app, OR you are targeting .NET 4, changing the certificate will not cause you any problems.

If you're after a quick solution, then you can "renew" your existing certificate and just give it a longer expiry date.

Cliff Stanford has cleaned up the Microsoft "workaround" and made it available as a simple command line exe - available here: http://may.be/renewcert/ - Nice work Cliff !