Extend Key or Subkey
The default for keys created in GPG Keychain is to have an expiration date 4 years after their creation. The expiration date serves as a safety measure. Should you loose your secret key or password and get into a situation unable to revoke the key, the key will then expire by itself once that date is reached.
Since GPG Suite 2018.4 (released 2018-09-21) GPG Keychain and GPG Mail will warn users four weeks prior to an expiration. You can then easily extend your key for another two years with a single click.
Even in cases where your key has expired but you still want to use that key and have access to both the secret key part and the password, you can easily extend your 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.