Unscrew the bolt at the center of the head-stem where the stem is connected to the headset. This is the point where the handlebars pivot on the stem.
Unscrew the bolts connecting the handlebars to the head-stem. Note the location of the brake cables. Loosen and remove them.
Slide the existing stem off the headset. Slide the new riser stem onto the headset.
Replace the handlebars and bolts onto the new riser stem. Replace the brake cables in the correct positions.
Replace and tighten the bolts at the center of the head-stem. Tighten everything securely and then test-ride your bike to ensure a quality fit.