// get_posts in same custom taxonomy
$postlist_args = array(
	'posts_per_page'  => -1,
	'orderby'         => 'menu_order title',
	'order'           => 'ASC',
	'post_type'       => 'lessons',
	'your_custom_taxonomy' => 'curs'
);
$postlist = get_posts( $postlist_args );

// get ids of posts retrieved from get_posts
$ids = array();
foreach ($postlist as $thepost) {
	$ids[] = $thepost->ID;
}

// get and echo previous and next post in the same taxonomy
$thisindex = array_search($post->ID, $ids);
$previd = $ids[$thisindex-1];
$nextid = $ids[$thisindex+1];
if ( isset($previd) ) {
	echo '<a rel="prev" href="' . get_permalink($previd). '">previous</a>';
}
if ( isset($nextid) ) {
	echo '<a rel="next" href="' . get_permalink($nextid). '">next</a>';
}
?>