Sharepoint Content Query Web part selective subsite

Question

Dear SharePoint Expert,

I want to create a Content Query web part in SharePoint 2013 called "Latest Upload":

 1. i created the web-part:
 2.in query  source: i selected --> Show items from the following site
 and all sub-sites: i chose a site.
 3.what i need to do is to exclude some of the folders or sub-sites that the query is retrieving data from.

**How can i accomplish that?**