When you say "Rotation point", you mean the origin of the rotation right?
If it's the case, you need to place a Node2D where you want your origin point of rotation and attach your TextureButton in this Node2D.
-> TextureButton2 ...
With this, you can move around your Node2D (and everything attached to it) to change where your rotation start with
if(event.type == InputEvent.KEY):
if(event.scancode == KEY_V) && (event.pressed):
## Everytime you gonna press V, the rotation will add +0.5 to the current rot position, you can change it to make it smoother or faster.
If you want to change the location of your rotation origin, just move around the Node2D like this (the movement won't be smoother with this method):
get_node("Node2D").set_pos( Vector2(x, y) )
Don't know if it's what you would like, but eh.