How to change image pop up depending on health percentage

Hello, what I want to do is if my player health goes below 50% I have an image that pops up and informs the player they are dying it also has some cool blood effects. What I want to happen is if the Player picks up a health pick up his health is regenerated and the image pop up goes away. The problem I am having is that the image pop up is not showing up when health falls below 50% and if I can get the image pop up to show up I can only have it disappear if I have exactly 51% health. What am I doing wrong or what am I missing? Any help would be greatly appreciated. here is my code.

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

public class PlayerHealth : MonoBehaviour
{
[SerializeField]
Slider healthBar;
[SerializeField]
Text healthText;
[SerializeField]
GameObject DeathUI;

[SerializeField]
private string sceneNameToLoad;

public float maxHealth = 100;
public float curHealth;

void Start()
{
	healthBar.value = maxHealth;
	curHealth = healthBar.value;
}

void  OnTriggerStay ( Collider other  )
{
	if (other .gameObject.tag == "Damage")
	{
		healthBar.value -= 1f;
		curHealth = healthBar.value;

	}

	if (other .gameObject.tag == "Heal")
	{
		healthBar.value += 1f;
		curHealth = healthBar.value;

	}

	if (other .gameObject.tag == "HPickUp")
	{
		healthBar.value += 25f;
		curHealth = healthBar.value;
	}

}

void Update()
{
	healthText.text = curHealth.ToString () + " %";

	if (curHealth <= 51) {
		DeathUI.gameObject.SetActive (false);
	}
	    else if (curHealth <= 50)
		DeathUI.gameObject.SetActive (true);

	if (curHealth <= 0)
		SceneManager.LoadScene (sceneNameToLoad);

}

}

Change the if statements to these:

if (curHealth >= 51)
         DeathUI.gameObject.SetActive (false);
else
         DeathUI.gameObject.SetActive (true);

Your if statements logic are wrong. You want the image to disappear when your health is greater or equal to 51, not less than or equal to 51.