Change Previous / Next buttons in Portfolio items

have you looked to the date of that question ( and solution) ?

Ok – try this in child-theme functions.php:

function my_avf_post_nav_entries( array $entries, array $settings, array $queried_entries ){
  $next = $entries['next'];
  $entries['next'] = $entries['prev'];
  $entries['prev'] = $next;
  return $entries;
}
add_filter( 'avf_post_nav_entries', 'my_avf_post_nav_entries', 10, 3 );

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Off-Topic

to have the same category this would be nice to have:

function my_avf_post_nav_settings( array $settings ){
  if( true === $settings['is_hierarchical'] ){
    $settings['skip_output'] = true;
    return $settings;
  } 
  if( ! in_array( $settings['type'], array( 'post', 'portfolio' ) ) ){
    $settings['skip_output'] = false;
    return $settings;
  }
  $settings['same_category'] = true;
  $settings['is_fullwidth'] = false;
  $settings['skip_output'] = false; 
  return $settings;
}
add_filter( 'avf_post_nav_settings', 'my_avf_post_nav_settings', 10, 1 );