How does oxidation state change?
It changes appropriately..........
What do I mean? Well, in a chemical reaction, ALL chemical reactions, 2 things are conserved: (i) mass; and (ii) charge. We introduce electrons as virtual particles to help us conserve both mass and charge. Electrons are presumed to transfer FROM the species oxidized, and TO the species reduced; they are invoked as virtual particles in order to help us balance the charge.
See this old answer for details and examples.