To find any expired repository keys and their IDs, use
apt-key as follows:
LANG=C apt-key list | grep expired
You will get a result similar to the following:
pub 4096R/BE1DB1F1 2011-03-29 [expired: 2014-03-28]
The key ID is the bit after the
BE1DB1F1 in this case.
To update the key, run
sudo apt-key adv --recv-keys --keyserver keys.gnupg.net BE1DB1F1
Note: Updating the key will obviously not work if the package maintainer has not (yet) uploaded a new key. In that case there is little you can do other than contacting the maintainer, filing a bug against your distribution etc.
One liner to update all expired keys: (thanks to @ryanpcmcquen)
for K in $(apt-key list | grep expired | cut -d'/' -f2 | cut -d' ' -f1); do sudo apt-key adv --recv-keys --keyserver keys.gnupg.net $K; done