wpmem_inc_login_inputs

/**
 * Example changes username field label to 
 * just "Email".
 * 
 * Anything in the [0] array position is the
 * username field, the [1] position is the 
 * password.
 */
add_filter( 'wpmem_inc_login_inputs', 'my_login_inputs' );
function my_login_inputs( $default_inputs ) {

    $default_inputs[0]['name'] = 'Email';

    return $default_inputs;
}