There are (at least) two kinds of vertical line spacing. Intra-paragraph line spacing, and Inter-paragraph line spacing. Both are paragraph-level formatting best set in Styles.
To do it directly (not through styles) right-click within the paragraph you want to change and select "paragraph." Set the line spacing you want. This is the Intra-paragraph line spacing, - the spacing between lines when you just keep typing and the text wraps
to the next line. You can also change the spacing before and after the paragraph.
To change this for new documents or easily apply it to all the text in your document, you would want to modify the paragraph style in which your text is typed. If the style is on the Quick Styles in your Home ribbon, right click on it and select modify. Then
click the Format button and select paragraph. Again set the line spacing and paragraph spacing options.
Tell Word you want this to apply to new documents based on this template to save it.
