Given that your key has expired.
$ gpg --list-keys
$ gpg --edit-key KEYID
Use the expire command to set a new expire date:
gpg> expire
When prompted type 1y or however long you want the key to last for.
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" when using they key:
gpg> trust
Test it out, do a round trip:
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 public key:
gpg --keyserver keyserver.ubuntu.com --send-keys KEYID
gpg --keyserver pgp.mit.edu --send-keys KEYID