Typically, you would go 20mm (2 cm) longer/shorter. 10mm (1cm) won't make that much of a different. However, be careful as you can affect your reach and therefore comfort on the bike. If your stem is currently "un-flipped", i.e. the angle of the stem (if your stem has a rise to it) is pointing up, then "flipping" it, or turning it upside down so that the angle points down, might give you the more aggressive posture you want without affecting your reach. If you stem has 0 degrees of rise, then flipping it won't make a difference.

I just "un-flipped" my stem because I felt the reach was long and I wanted a more upright position. My 110mm stem only has a 6 degree rise so the difference was not much different.