Selecting an element from a new window using Selenium Java

I have a question. I want to select an element from a new window. What happens in my application is when I select a menu item called 'Document Upload' it will launch a small window on top of the current window that will allow me to either drag and drop files or select a browse button so that I can access files from my filesystem. I want to be able to interact with the second window that is displayed on top of my current window. Any suggestions?

Swithch the control to 2 nd window.
and upload the docs