This following article will show you how to set the default 'Send from' address for your site.
Step One: Navigate to settings
Within the admin section, navigate to 'Settings' from the global navigation
Step Two: Navigate to Email
From the left-hand menu select email. You should now see the option to set a From address, enter the address you would like to be your default.
Step Three: Submit and save changes
Scroll to the bottom of the page, and click the submit button to save the changes made