List your secret keys, and decide which one to export.
gpg --list-secret-keys --keyid-format LONG
Export the secret and public parts of the key.
gpg --export --armor <key-id> > gpg-pub.asc
gpg --export-secret-keys --armor <key-id> > gpg-sc.asc
Import the parts.
gpg --import gpg-pub.asc
gpg --import gpg-sc.asc