Virtual Button on vuforia to increment percentage

Hi, I am trying to make two virtualbuttons on an image target, this two button are minus (-) and plus (+) and y want that these two buttons could increment a number in 3dtext but i do not know how. Any ideas? I have this code:

using UnityEngine;

public class increment : MonoBehaviour, IVirtualButtonEventHandler
{
/// <summary>
/// Called when the scene is loaded
/// </summary>
void Start() { }

/// <summary>
/// Called when the virtual button has just been pressed:
/// </summary>
public void OnButtonPressed(VirtualButtonAbstractBehaviour vb) { }

/// <summary>
/// Called when the virtual button has just been released:
/// </summary>
public void OnButtonReleased(VirtualButtonAbstractBehaviour vb) { }
}