Principle of Transmissibility

Assume Force $\overrightarrow{F}$ is applied to a rigid body at point $A$.
Apply equal and opposite forces $\overrightarrow{F}$ and $-\overrightarrow{F}$ at point $B$.
$\overrightarrow{F}$ at point $A$ cancels $-\overrightarrow{F}$ at point $B$.
Force $\overrightarrow{F}$ has been translated from point $A$ to point $B$.
Thus the point of application of a force can be translated anywhere along its line of action without changing the external reaction forces acting on the body.