Change portfolio slug
Go into /wp-content/plugins/krown-portfolio/includes/portfolio.php and change line 41 to whatever you want:
'rewrite' => array("slug" => "portfolio"), // Permalinks format
!! NOTE 1 : After the change you would have to go to Settings > Permalinks and change the permalinks format back to default, do a few refreshes, then change it back to the one suggested in the manual. This is for flushing the old one.
!! NOTE 2 : Because you are changing permalinks, all the projects that you have created using the old permalinks will be gone! They will be restored when you change the permalink, of course.