You can enable the SEO/SEF feature in Opencart through the admin area > System > Settings > Edit > Server. Select "Yes" for the option "Use SEO URL's:" and click "Save".

This will generate an .htaccess.txt file with the mod_rewrite rules in your Opencart root folder. You will have to rename that file to .htaccess in order to enable the feature completely.

After that clear your browser's cache and refresh the site to see the new links.