Slider.maxValue

public float maxValue ;

Description

The maximum allowed value of the slider.

using UnityEngine;
using System.Collections;
using UnityEngine.UI; // Required when Using UI elements.

public class Example : MonoBehaviour
{
    public Slider mainSlider;

    void Start()
    {
        // Changes the max value of the slider to 20;
        mainSlider.maxValue = 20;
    }
}