Given that your key has expired.
$ gpg --list-keys
$ gpg --edit-key KEYID
Use the expire command to set a new expire date in the far future, e.g. 1y.
gpg> expire
Select all the subkeys (the primary key, which we just set the expires date for, is key 0):
gpg> key 1
gpg> key 2
gpg> expire
A star will sppear before all selected keys.
Since the key has changed we now need to trust it, otherwise we get the error ""There is no assurance this key belongs to the named user":
gpg> trust
Test it out, create an encrypted message.
gpg -ea > secret.out
gpg -d secret.out
Backup the key:
gpg -a --export KEYID > kris.leech.gpg.public
gpg -a --export-secret-keys KEYID > kris.leech.gpg.private
Move the keys on to something like a USB drive and store it safely in another location.
Publish the key:
gpg --keyserver keyserver.ubuntu.com --send-keys KEYID
gpg --keyserver pgp.mit.edu --send-keys KEYID
If you need to copy/paste the key to a public server:
xclip -i kris.leech.gpgp.public -selection clipboard