Simple Drag and Drop in Unity

MouseInput.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class MouseInput : MonoBehaviour {

    Vector3 offset = new Vector3 (0, 0, 10);
    bool follow = false;
    void OnMouseDown () {
        follow = true;
    }

    void OnMouseUp () {
        follow = false;
    }

    void Update () {
        if(follow)
        transform.position = Camera.main.ScreenToWorldPoint (Input.mousePosition) + offset;
    }
}