Do you mean the front or rear sprocket? According to the Nexus info, the rear hub has options for 16, 18, 19, 20, 21, 22, or 23 tooth sprockets. You could swap out the rear sprocket for something larger to get lower gears. Or you could switch to a smaller front chainring - though there's a good chance your crankset is not made to swap chainrings. Do you use all of your high-end gears? If you currently use all of the high gears, then swapping the sprocket will give you more at the low end but less at the high end. If you never use all the high gears, then swapping sprockets would work for you. Swapping the rear sprocket will be a cheaper solution.