Modify struct members the same way you modify regular variables. Access with the dot operator, then assign new values. The syntax is exactly what you expect. Example: Point p; p.x = 5; p.y = 10; creates a Point and sets coordinates.
Later change them: p.x = 100;. The member x updates, y stays the same. You can read, modify, pass to functions, anything you do with normal variables. p.x++ increments x. p.y *= 2 doubles y.
Members behave like they always did.