how to start coroutines in the update method?

hello! I was wondering if there were any way to start a coroutine in the update method using something like yield return StartCoroutine("reload"); but I would like to declare this in the update method in an if statement.... however, the problem is the update method "cannot be an iterator block because 'void' is not an iterator interface type" and I can't fix this because Update can not be declared using IEnumerator which is what you usually do for coroutines. please help! here is the compile error i am getting:

error CS1624: The body of 'RayCast.Update()' cannot be an iterator block because 'void' is not an iterator interface type

here is my code:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 public class RayCast : MonoBehaviour
 {
     public ParticleSystem MuzzleSmoke;
     public ParticleSystem MuzzleFlash;
 
     public int Bullets = 500;
     public bool CanFire = true;
     Camera cam;
 
     private void Start()
     {
         
 
         CanFire = true;
         cam = Camera.main;
       
     }
 
     void Update()
     {
 
 
 
         Ray ray = cam.ScreenPointToRay(Input.mousePosition);
         if (Input.GetMouseButton(0))
         {
 
             if (Bullets <= 0)
             {
 
                 CanFire = false;
 
             }
 
             if (CanFire == true)
             {
 
                 Bullets -= 1;
                 MuzzleFlash.Play();
                 MuzzleSmoke.Play();
 
                 if (Physics.Raycast(ray, out RaycastHit hit))
                 {
                     hit.collider.gameObject.transform.GetComponent<EnemyHealth>().HEALTH -= 1f;
                 }
 
             }
 
 
             if (Input.GetKeyDown(KeyCode.R))
             {
                 if (Bullets <= 0)
                 {
                     yield return StartCoroutine("reload");
                 }
             }
 
         }
 
         IEnumerator reload()
         {
 
             yield return new WaitForSeconds(5);
 
             Bullets += 500;
             CanFire = true;
 
 
         }
 
        
         
 
 
     }
 
 }