Extending the expiry on a GPG key is not very hard, but it's easy to forget a step. Here's how I did my last expiry bump.

Update the expiry on the main key and the subkey:

gpg --edit-key KEYID
> expire
> key 1
> expire
> save

Upload the updated key to the keyservers:

gpg --export KEYID | curl -T - https://keys.openpgp.org
gpg --keyserver keyring.debian.org --send-keys KEYID