gpg --list-secret-keys --keyid-format=long
gpg --armor --export YOUR_KEY_ID
gpg --armor --export-secret-keys YOUR_KEY_ID > encrypted_private_key.asc
gpg --import encrypted_private_key.asc
Run gpg --edit-key YOUR_KEY_ID
Run gpg> list
to see user IDs
Run gpg> uid N
to select the user ID you want as primary (replace N with the number)
Run gpg> primary
Enter your key's password when prompted
Run gpg> save
to save changes