A fast way to do this is to go to the tool bar and click on <Effects> then <Colour Filter> then <Hue/Saturation/Brightness.  Take the slider on the Saturation scale and slide it all the way over to the left (-100).  You'll decolourize the photo.  Then just hit "Apply".

Note as well that the user manual indicates you can decolourize a photo by going to the <1 Click> then <Effects and Filters> then <Transform into Painting>.  There is a drop down box at the top asking you to select a desired effect - click on there and choose "Newspaper Print".  This will also decolourize your photo but it appears to me to be pixelated more than if you use the Saturation technique above.