For changing the slug, replace the line:
'rewrite' => true,
at otw-portfolio-light.php inside $args = (.....); for register_post_type( 'otw-portfolio', $args ); with:
'rewrite' => array('slug' => 'new-slug'),
note:
- this will be overridden on plugin update
- additional you might have to go to Dashboard > Settings > Permalinks and save to make changes appear