first script:

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

public class highscore : MonoBehaviour {

    public Text bestscore;

    // Use this for initialization
    void Start () {
        bestscore.text = PlayerPrefs.GetFloat ("BestScore", 0).ToString (0.0);
    }

    // Update is called once per frame
    void Update () {

    }
}


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Second script:
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using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class HUD : MonoBehaviour {

    float maxHealth = 100f;
    public static float health;

    public Text timeText;
    private float startTime;

    // Use this for initialization
    void Start () {

        startTime = Time.time;
    }

    // Update is called once per frame
    void Update () {

        float t = Time.time - startTime;

        string minutes = ((int)t / 60).ToString ();
        string seconds = (t % 60).ToString ("f0");
        timeText.text = minutes + ":" + seconds;
    }

    float currentBestScore = PlayerPrefs.GetFloat("Bestscore", 0);
    float currentScore = Time.time - startTime;

    if (currentScore < currentBestScore)
    {
        PlayerPrefs.setFloat("BestScore", currentScore);
    }

}