Support: http://kopatheme.com/forums/topic/how-do-i-change-next-article-to-say-next-service-or-client-or-staff/

post-navigation.php

<?php 
$previous_title = __( 'Previous Article', kopa_get_domain() );
$next_title = __( 'Next Article', kopa_get_domain() );

if ( 'staffs' === get_post_type() ) {
    $previous_title = __( 'Previous Staff', kopa_get_domain() );
    $next_title = __( 'Next Staff', kopa_get_domain() );
} elseif ( 'clients' === get_post_type() ) {
    $previous_title = __( 'Previous Client', kopa_get_domain() );
    $next_title = __( 'Next Client', kopa_get_domain() );
} elseif ( 'testimonials' === get_post_type() ) {
    $previous_title = __( 'Previous Testimonial', kopa_get_domain() );
    $next_title = __( 'Next Testimonial', kopa_get_domain() );
} elseif ( 'portfolio' === get_post_type() ) {
    $previous_title = __( 'Previous Portfolio', kopa_get_domain() );
    $next_title = __( 'Next Portfolio', kopa_get_domain() );
} elseif ( 'services' === get_post_type() ) {
    $previous_title = __( 'Previous Service', kopa_get_domain() );
    $next_title = __( 'Next Service', kopa_get_domain() );
} 

$prev_post = get_previous_post(); 
if ( ! empty( $prev_post ) ) :
?>
    <p class="prev-post">
        <a href="<?php echo get_permalink( $prev_post->ID ); ?>">&laquo;&nbsp;<?php echo $previous_title; ?></a>
        <a class="article-title" href="<?php echo get_permalink( $prev_post->ID ); ?>"><?php echo $prev_post->post_title; ?></a>
        <span class="entry-date"><?php echo get_post_time( get_option( 'date_format' ), false, $prev_post->ID ); ?></span>
    </p>
<?php endif; ?>

<?php
$next_post = get_next_post();
if ( ! empty( $next_post ) ) : 
?>
    <p class="next-post">
        <a href="<?php echo get_permalink( $next_post->ID ); ?>"><?php echo $next_title; ?>&nbsp;&raquo;</a>
        <a class="article-title" href="<?php echo get_permalink( $next_post->ID ); ?>"><?php echo $next_post->post_title; ?></a>
        <span class="entry-date"><?php echo get_post_time( get_option( 'date_format' ), false, $next_post->ID ); ?></span>                                
    </p>
<?php endif; ?>