Can impulse be decreased?
Either decrease the amount of force you apply to something or the amount of time over which you apply the force.
Impulse is a change in momentum.
Usually you don't change the mass of something, you change its velocity by applying a force to cause and acceleration.
You can think of impulse as the area under a force vs time graph.
Or to use calculus:
If you want to decrease the area in the triangle above, you have to make either the height of the triangle smaller (decrease the average force) or the base smaller (decrease the time interval).