Given that your key has expired. Find and edit the key:
$ gpg --list-keys $ gpg --edit-key KEYID
Use the expire command to set a new expire date:
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:
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
If you need to copy/paste the public key to a key server:
xclip -i kris.leech.gpg.public -selection clipboard