A ball drops down, how to reverse the velocity?

First you'll have to detect if it's hitting the ground. You can use OnTriggerEnter for that. If the ground is parallel to the x-axis you'd just need to reverse the y component of your velocity. If you need a more general approach you can use Vector3.Reflect.

You can also try adding a rigidbody with gravity to your ball, and giving it 'bouncy' physics properties so it would bounce naturally.