Change the line spacing in an entire document
Go to Design - Paragraph Spacing.
Choose an option. To single space your document, select No Paragraph Space.

Change the line spacing in a portion of the document
Select one or more paragraphs to update. Press Ctrl + A to select all.
Go to Home - Line and Paragraph Spacing Line and Paragraph Spacing button
Select Line Spacing Options and choose an option in the Line spacing box.

Adjust the Before and After settings to change spacing between paragraphs.
Select OK.