I know this may sound absurd, but I can't figure out how to raise the handlebars on my Bianchi Boardwalk (2005, I think). I've loosened the two allen nuts so I can adjust them laterally. i.e. turning them to align them to the wheel, but I can't raise them.
If the stem is a threadless model with two pinch bolts holding it to the steerer tube...you can't raise it.
Either purchase another stem with more rise or an adjustable model. There are also steerer extenders available but I generally don't recommend them, preferring to swap the bars for a higher rise type. In any case you may also need to replace the control cables.
Most shops will have the parts you'd need, which is a 1-1/8" threadless high rise or adjustable stem and perhaps a spacer as well as the cables if needed ($20-30 for a stem, cables $10-20 or so + labor). They can give you an idea what it would cost, what's needed, and if you bought your bike from them, they're likely to be a bit more flexible and accomodating to a return customer.