The eyepiece is in a tube that tapers down. At the leading edge (the end away from the eye) there is a knurled ring (closer to you than the magnification ring). Hold the knurled ring and turn the eyepiece bell counter clockwise. The knurled ring can then be backed off (towards the muzzle) and you will be able to screw the eyepiece bell in and out to get a good sight picture. The turn the locking ring clockwise to lock the bell in place.

Note it will a also change the "focus" of the crosshairs so you can't get much change before it will give a blurry crosshair image. You don't want that. Usually the plan is to adjust the scope in the rings and then fine tune with the procedure mentioned above to "focus" the crosshairs in the image field.