It is set up as default but of course there is way to do the single-space mode.
Whenever you want to apply single-space, just press Shift Key while you are hitting the Enter Key.
Then it turns to single-space mode.
So if you want to write your document in single-space, plz use Shift-Enter mode.
 
Example:
Hi there, (Enter)
 
Welcome to DreamManage! (Enter)
 
Hi there, (Shift-Enter)
Welcome to DreamManage!