Child Theme install

Hi,

I have installed the main theme by uploading the entire zip file through the WP interface. I don't understand how I to active the child theme? Do I need to use FTP? I read the information on this subject and I understand how it functions I just don't see how to get the file into WP. I guess it is actully already there but how do access it and activate it?

Thank you for your help

Cyndie