Remove the top cap.
Remove any spacers that might be in between the top cap and the stem.
Loosen the 2 allen bolts on the stem clamp.
Remove the stem.
Remove the amount of spacers below the stem equal to the amount you want to lower your handlebar by.
Replace the stem.
Replace the spacers that you removed from below and above the stem and put them all back in place.
Put the top cap on.
Adjust the preload on the top cap.
Adjust the handlebar.
Lock down the 2 allen bolts on the stem.
These instructions are assuming that you have spacer(s) below the stem that you can remove to lower the handlebar.