– selection of the unit(s)
– selection of a hex/location OR selection of enemy unit(s)
In your case, select the stack you want to move. Select/deselect units from the stack in the left hand side Counter Inspector until you have only those units selected you want to move. Then (or before, the order is not important) select the hex (and/or location in case of upper building levels) you want them to move (a location other than the ground level location can be selected in the right hand Hex Inspector). Hover over the “Move” button to find out what is missing/wrong to get a tooltip OR if everything is alright, hit it.
All this is only possible in the Movement Phase of course.