how to use input.getkeydown in unity code example

Example: how to get key input in unity

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    void Update()
    {
        if (Input.GetKey("up"))
        {
            print("up arrow key is held down");
        }

        if (Input.GetKey("down"))
        {
            print("down arrow key is held down");
        }
    }
}