Live stretching & Set new design
You can use Set new design on the Button & NavBar Tool to replace the existing button for the selected state. If the existing button is off-center, Live stretching (also on the Button & NavBar Tool) controls the position of the new design.
For example, if the mouse-off state looks like this:
and the mouse-over state looks like this (the dotted outline shows the mouse-off state):
then, with Live stretching selected, Set new design for the mouse-over state positions the new design in the center of the button (that is, in a different position to the original mouse-over design):
With Live stretching deselected, the new design is in the same position as the original design: