Cut a split on the inside of the waist band, or find the seam and open it.
Pull out the elastic, just far enough to cut off two inches.
Cut the elastic, and sew in back into one continuous band.
Let the elastic go back into the waistband.
Sew up the seam or the slit.
If the elastic itself is stitched throughout its length, it will have to be unpicked, shortened and restitched, That, or the side seams will have to be restitched.