Update Entity Appengine Datastore with Go

I'm trying to find an effective example in how to perform updates on appengine datastore with Go.
All the examples I've found on the web are very vague and mostly explains concepts and not the "real life".
The appengine documentation for go says:

..."Updating an existing entity is a matter of performing another Put() using the same key."

My problem here is being in how to retrieve the key. So I have the code below to store and retrieve data:

func subscribe(w http.ResponseWriter, r *http.Request) {
	
	user := User {
		Name: r.FormValue("username"),
		Email: r.FormValue("useremail"),
		Flag: 0,
	}
	
	c := appengine.NewContext(r)
	//datastore.Put(c, datastore.NewIncompleteKey(c, "User", nil), &user)
	datastore.Put(c, datastore.NewKey(c, "User", "stringID", 0, nil), &user)
	
	template.Must(template.ParseFiles("confirmation.html")).Execute(w, nil)
	
}

func checkusers(w http.ResponseWriter, r *http.Request) {
	
	c := appengine.NewContext(r)
	
	qUsers := datastore.NewQuery("User")
	
	var users []User
	
	qUsers.GetAll(c, &users)
	
	template.Must(template.ParseFiles("users.html")).Execute(w, users)
}

How do I do an update on the flag property changing its value tom 1?

I'm a bit confused on this thing as I couldn't fully understand how the "key" is stored for each entity.

Any help would be very appreciated.

todo an update you first need to identify if your object is a new or an old one.
this can be simple done by adding the following method to your User struct:

type User struct {
    Name string
    Email string
    Flag int64  `datastore:"-"`
}
func (u *User) IsNew() bool {
	return u.Flag == 0
}

this tells data-store to ignore the Flag field when storing and retrieving an object
and because initial value of int64 is zero, a newly created object can be identified if Flag is zero

so creating a new object just needs to set UserName and Email:

user := User {
    Name: r.FormValue("username"),
    Email: r.FormValue("useremail")
}

then next step is to either use a IncompleteKey or a Key, for the put statement

could look like this:

var k *datastore.Key
if user.IsNew() {
	k = datastore.NewIncompleteKey(c, "Users", nil)
} else {
	k = datastore.NewKey(c, "Users", "", user.Flag, nil)
}
k, err := datastore.Put(c, k, user)
if err != nil {
	return k, err
}

with an incomplete key, app-engine will generate a new key for you.
after put you can assign the new key to your object:

user.Flag = k.IntID

now if you do a query later you need to assign the Id to your query result objects,
the query will return the keys of query result in the same order so you can change your code like this:

keys, err := q.GetAll(c, &users)
if err != nil {
	return
}
l := len(users)
for i := 0; i < l; i++ {
	users[i].Flag = keys[i].IntID()
}

thats all, for more information, just have a look a the reference document there is explained with methods return which values.
https://developers.google.com/appengine/docs/go/datastore/reference