A good place to do this is at the knee line, where you often already have a marking/ notch. 

Make the adjustment at the same place on both front and back. Draw a straight line at 90 degrees to the grainline on front and back. and cut the pieces apart along the line.

To shorten, overlap the pattern pieces with how much you would like to shorten the leg length. 

To lengthen, move the pieces apart by the desired distance and add some paper with some tape in between. Connect the two lines on the pieces. 

draw a line and cut the pieces                                    lengthen the leg                                  shorten the leg