Extend Key or Subkey
The default for keys created in GPG Keychain is to expire four years after creation. That measure serves as a safety not in case the secret key or password is lost. In a situation, in which you are unable to revoke the key, the key will then expire by itself once that date is reached.
Starting with GPG Suite 2018.4 (released 2018-09-21) GPG Keychain and GPG Mail users will see a warning, four weeks prior to an expiration. You can then easily extend your key for another two years with a single click.
Should your key already be expired that is not a problem - how to extend your expired key:
Double click your sec/pub key and in the Key tab click Change… Modify the date as desired. After extending your key, you will be asked if you want to upload your updated public key.
In the Subkey tab right-click the Subkey and select Change Expiry Date. Modify the date as desired. After extending your Subkey, you will be asked if you want to upload your updated public key.
After extending your key or subkey you are asked, wether you want to upload your updated public key. This is an important step, as otherwise your friends won't be able to know, that your key has been extended. In case you do not want to use the key servers, make sure to email your updated key directly to your contacts.