Changing Force of object

I need help trying to change the force of an object at a certain point. For example (-22.5, 0, 0). It would be a real help if you can help me with my problem.

if you want calculate force : if the gravity on your object is on it alter a force with formula f=MG that G can have (G.x,G.y,G.z) then you can compute your force like this : F = (-22.5,0,0)-(MG.x,MG.y,MG.z)
at default G = (0,-9.81,0) M = Mass , G = Gravity

if you want script :
see Unity - Scripting API: ConstantForce
and Unity - Scripting API: Rigidbody.AddForce