sinchy
Ctr + A to highlight your whole document
Then you click on "Format" (one of the File, Edit, etc buttons)
Look down the list and click on "Reveal Formatting"
Another box sohuld pop up.
Underneath the section labelled "Paragraph", click on "Indentation" and then another box should pop up.
In the tab "Indents and SPacing" look around for a lil box that says "Line Spacing"
And there you can select "Double"
hope it helps/makes sense!