Leftclick on Unit B and then keep on pressing the button "alt" on you keyboard. By still pressing "alt" your rightclick on unit A (at me it doesn't work like that, I have to rightclick on a free hex field close to unit A and then on the unit A). You see that it works, if the white arrow shows, that unit B will move on place occupied by unit A.

Edit: You can combine the orders with targeting a special enemy unit by keep on pressing "ctrl" (or on german keyboards: "strg") and rightclick on enemy unit you want to attack. Doesn't work like intended, if the enemy unit dies before. As always you see the order is accepted by the red arrow appearing.