Just googling around to find how to rotate camera with mouse click and drag. Thanks to this link: https://answers.unity.com/questions/1189946/click-and-drag-to-rotate-camera-like-a-pan.html

This code is originally taken from that link:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
public class CamRotate : MonoBehaviour {
public float speed = 3.5f;
private float X;
private float Y;
void Update() {
if(Input.GetMouseButton(0)) {
transform.Rotate(new Vector3(Input.GetAxis("Mouse Y") * speed, -Input.GetAxis("Mouse X") * speed, 0));
X = transform.rotation.eulerAngles.x;
Y = transform.rotation.eulerAngles.y;
transform.rotation = Quaternion.Euler(X, Y, 0);
}
}
}

Then I made a slight changes to clamp vertical camera rotation:

if(Input.GetMouseButton(0)) {
transform.Rotate(new Vector3(Input.GetAxis("Mouse Y") * speed, -Input.GetAxis("Mouse X") * speed, 0));
X = transform.rotation.eulerAngles.x;
if (X > 60.0f && X < 70.0f)
X = 60.0f;
if (X < 300.0f && X > 290.0f)
X = 300.0f;
Y = transform.rotation.eulerAngles.y;
transform.rotation = Quaternion.Euler(X, Y, 0);
}