Put the pants on inside out, pin the crotch to the desired length. You should leave approximately a half inch of space between the leg and pin. Now run the pants through a sewing machine on the outer side of the pins starting on the edge of the original hem.
Starting at the mid-thigh of the inner seam, use a sewing machine to stitch a new inseam. Stitch upward toward the crotch, gradually making a wider seam in the crotch area, then tapering off down the other side.
This is actually pretty complicated, and unless you sew frequently, then you may not want to try it. You need to let out the seams, removed material from the waist, and sew back.