How can momentum be decreased?
Decrease velocity and/or decrease mass.
Momentum of a object is equal to its mass multiplied by its velocity.
p = mv
(Momentum is a vector quantity because velocity is a vector).
Decreasing either mass or velocity will decrease momentum.
Some or all of the momentum of an object can be transferred to another object by colliding with it.
Momentum is conserved during a collision unless some work is done (eg permanently bending something, generating sound, etc).