This DaycountManager takes care of changing the DifficultyLevel variable.

DayCountManager.cs

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

public class DayCountManager : MonoBehaviour
{
    //This is the current day. We set it at 1 at the start
    [HideInInspector]
    public int CurrentDay;
    //We make a Populator_Manager reference so we can drag the Populator_Manager and use its difficulty function
    public Populator_Manager Populator;
    
    void Start(){
        CurrentDay = 1;
    }
    void Update(){
        Debug.Log("Day:"+CurrentDay);
    }
    public void NextDay(){
        
        //The previous script, SleepClickable, calls the NextDay() function to move to the next day
        //When it does this, it checks if it's between a certain set of numbers, and then we change the
        //Difficulty Level accordingly
        if(CurrentDay > 1 && CurrentDay <= 3){
            Populator.ChangeDifficultyLevel(2);
        }

        if(CurrentDay > 3 && CurrentDay <= 10){
            Populator.ChangeDifficultyLevel(3);
        }
        //We then call the Populator to reset the Dynamic Objects in the world
        //Reseting the zombies and the crates in the world
        Populator.ResetDynamicObjects(); 
        //We then add 1 to the current day
        CurrentDay += 1; //Add a day to the counter

      
    }


}