Plugin: WP Single Post Navigation -- Customize parameters for "next post link" -- add this to functions.php of your theme or child theme

gistfile1.aw

<?php

add_filter( 'wpspn_next_post_link', 'custom_wpspn_next_link' );
/**
 * WP Single Post Navigation: Add custom filters for "next post link"
 */
function custom_wpspn_next_link() {

	$args = array (
		'format'       		=> '%link',     // Change link format
		'link'                	=> '&laquo;',   // Change link string
		'in_same_cat'        	=> FALSE,       // Apply only to same category (default: FALSE)
		'excluded_categories' 	=> ''           // Exclude categories (default: empty)
	);

	next_post_link( $args['format'], $args['link'], $args['in_same_cat'], $args['excluded_categories'] );
}