Undo and take off the nut at the top. Pull the knob up and off. Put it back on in the zero position. Put the nut back on. The readings that are asending verticaly can't be put back to zero. So sight in for 100m or whatever take the turret off and put back to zero and record the vertial reading 2,3,4, or whatever is says. Then you can zero for 200m and record the readings to repeat later on.
If you were thinking that you could zero the rifle then put all the readings back to zero it doesn't work that way.