You can display the author’s display name, inside the loop, by using the following code snippet:

<?php echo 'Author: ' . get_the_author_meta( 'display_name' ); ?>

If you want to display the author’s display outside the loop, use the following code snippet:

<?php echo 'Author: ' . get_the_author_meta( 'display_name', $user_id ); ?>

Replace $user_id with the ID number of the user.

You can change the text before the author’s display name, by editing the content inside the single quotes of 'Author: '. If you wish to add text after the author’s display name, add . 'Ending text goes here' just before the semi-colon.