Hi Jayne, it'll depend on what version of Office you're using, but they're all more or less the same.
Click on the arrow next to the button that allows you to raise or lower the number of spaces between lines. (It has four horizontal lines and two arrows pointing up and down on the left and one single one on the right). Click on the one on its own and it'll give you a list of numbers and the word 'more'. Click on more and on the Indents and Spacing page select Line Spacing and single from the drop-down box.
Should work.