You can quickly set lines to be single-spaced or double-spaced by using the Spacing pulldown menu in the ruler. See using the ruler.

For more specific line spacing, select the text you want to affect, and then choose Format ‣ Baseline ‣ Spacing….

A dialog appears with line spacing options, the most important of which is “Line height multiple”. Adjust these settings as required, noting the appearance in the background.

Line spacing options