About WaitForSeconds

I want to live a GameObject for some seconds before destroy it.I saw the code by c# like this in documentation:
IEnumerator Awake() {
print(Time.time);
yield return new WaitForSeconds(5);
print(Time.time);
}
how to use this code?
If I put it in my code like this:
void update(){

Awake();

}
It is not called.Noting happen!

If I put it like this
void update(){

yield return new WaitForSeconds(5);

}
There is a error like this :
error CS1624: The body of demo.Update()' cannot be an iterator block because void’ is not an iterator interface type

If I put it like this:
void update(){

yield new WaitForSeconds(5);

}
There is a red line .

How to use this WaitForSeconds?Thanks!

Or just:

Destroy(theGameObject,5);