its actually pretty simple; replace all of the "using" at the top with below

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Reflection;

using Oxide.Core;
using Oxide.Core.Configuration;
using Oxide.Core.Libraries.Covalence;
using Oxide.Core.Plugins;
using Oxide.Game.Rust.Cui;

using Rust;
using UnityEngine;
using UnityEngine.UI;

ur welben