how do i change the wheel collider stiffness with a script?

In my game, you can upgrade the tires. the real difference between tires is the stiffness on the wheel collider. I want the stiffness of the wheel to be changed by a script, but when I use the WheelCollider.forwardfriction.stifness , I get an error. I think you can also use WheelFrictioCurve. ive tried, but cant see the difference. how do i change the wheel collider stiffness with a script?

Try this:

//You may also want to change wheelCollider.sidewaysFriction
//BTW, the "Stiffness Factor" in the editor only seems to represent sidewaysFriction.stiffness
WheelFrictionCurve frictionCurve = wheelCollider.forwardFriction;
frictionCurve.stiffness = 5f;//whatver you want
wheelCollider.forwardFriction = frictionCurve;