https://withblue.ink/2020/05/17/how-and-why-to-sign-git-commits.html

If a key expires, you can update the expiration date as follows:

• gpg --list-secret-keys to list the secret keys and look up the key ID, alternatively gpg --list-keys
• gpg --edit-key <key-id> to open the gpg console
• run expire in the gpg console and follow the prompts to update the expiration date
• finally save and quit

You can type help in the console for other available commands.