Registering a fingerprint
Tap the Home Button
> the Apps icon 
> Settings > Fingerprints & security > Fingerprints.Enter your unlock sequence.
Place your finger on the Power/Lock button and follow the screen prompts until the fingerprint is registered.
Make sure that your fingers and the Power/Lock button are clean and dry.
Moisture, lotions, sweat, oils, cut or dry skin might affect the fingerprint recognition. Certain activities can also temporarily affect fingerprint recognition, including exercising, showering, swimming, cooking, or other conditions that change the characteristics of your fingerprint.
Cover the Power/Lock button completely. Don't press too hard or too quickly (hardly move your finger while scanning is in progress).
As you use the Fingerprint feature, it will continue to learn and improve recognition of your fingerprint.