How to change the image (texture) of a GUITexture in C#.

 public GameObject weapon;      // drag your GUITexture object here in inspector
 
 // When you want to change the texture...
 
 weapon.GetComponent<GUITexture>().texture = Resources.Load("sword") as Texture2D;       // This is for C#

but I don't understand how to access the Texture that is in my assets folder.

You want to create a folder called Resources inside your Assets folder and put your image(s) inside the Resources folder. (Capital R in resources is important)

Then you can use Resources.Load();

  using UnityEngine;
 using System.Collections;
 
 public class GUIController : MonoBehaviour {
 
 public Texture2D yourtexture;
 // Use this for initialization
 void Start () {
 }
 // Update is called once per frame
 void Update () {
 }
 void OnGUI()
 {
 GUI.DrawTexture(new Rect(10,10,60,60), yourtexture);
 }
 }