To leave a unit, select the unit and move the Attachable Model out of the unit using a Move Manoeuvre. Attachable models ignore the Unit Spacing rule with regards to the unit they are leaving. The unit that is left behind is not considered to have moved, and is free to perform its own move later in the phase.
Building an army? Create, validate and share your list with New Recruit, army builder for The Ninth Age and many more games.