How to Modify Author Page for all Users?

by

To get the data of the user displayed on the current author archive page you can use:

$author = get_user_by( 'slug', get_query_var( 'author_name' ) );
$user_info = get_userdata( $author->ID );

More info on what is returned: http://codex.wordpress.org/Function_Reference/get_userdata

From there you can use the user details you need:

<?php echo $user_info->user_firstname . ' ' . $user_info->user_lastname; ?>