Line spacing on a text document

I am editing a text document which I imported from Microsoft Word. The document has single spacing and I’d like to change it double spacing. I know this must the easiest thing in the world, but I can’t figure out how to do it. When I go into the Text menu and the “Spacing” section I get the following options.
Line Height Multiple
Line Height
Inter-line Spacing
Paragraph Spacing

But none of these accomplishes the more basic task of “Double-spacing” between the lines.
What am I missing?

See Preferences: Text Editing: Default Main Text Attributes.
On the Spacing tab, choose Double.
You may also set any other attributes for the project there.

This didn’t work. The document still appears in the window as single-spaced.

Preferences only affect new documents.

Make sure you select the text you want to change in the text view - line spacing will only affect selected text, so select all of the text in the text view if you want to apply the line spacing to all of it. Then go to Text > Spacing… and set the line height multiple to 2. (You can also do this via the ruler if you have the ruler open.)

All the best,

Thank you!