From Assets select Fleet.

In the left hand side of the screen select the unit that you want to remove the operator from.

On the bottom right make sure you are on the Crew Assignment tab.

Once you are on the Crew Assignment Tab, you will see the assigned crew member(s) in the lower right hand side of the screen.

To remove a crew member right click on their name and choose "Remove Selected Crew member"

You will then get a pop up box that asks if you want to add a Crew History.

Select Yes so that you will save all prior history for that person with the equipment.

You will then get a box showing the operator that was assigned and the dates they were originally assigned and the date you are removing them on.  You can then add any notes and save.

You can then add a new crew member as necessary.