Most tripods have a 1/4" thread attached to the centre column, which then screws into the base of the head (some may have a 3/8" thread, but the principle is the same).
Grasp the head firmly in one hand, the shaft firmly in the other, and rotate the head counter clockwise to remove the old head. If it has been in situ for some time, it may be stiff to rotate.
Once you have removed the old head, lubricate the threads on the centre column, then attach the new head, rotating this one clockwise.
If the old head will not unscrew under firm manual pressure, remove the centre column and place in a vice, packing it with wooden battens to protect it. Then attempt to unscrew the head, using a mole grip or similar if necessary.
on Sep 14, 2016