By left clicking the number that shows remaining movement under unit icon to the right you deselect one from the group and if you move you move anyone but the unit.
If you right-click it then, it reverses it and selects only that single unit. You can do that witha ny amoutn of units. If you have 3 deslected and you righ-click any of their selection bar it makes them all selected instead.
Or you can click them one-by-one to leave 1 or 2 selected but it is less convenient.
I hope it's clear.

Thank you very much. It is very clear.