Google app engine + Go + datastore + add / update / delete record

Here is how I insert records in to DataStore

type User struct {
    Id string
    Name string
    Pass string
    Email string
}

user := User {
    Id:     "4be0f045-6ce8-4d3e-751e-15bd84f2b925",
    Name:   "test",
    Pass:   "test",
    Email:  "",
}

datastore.Put(context, datastore.NewIncompleteKey(context, "User", nil), &user)

How do I update or delete a record ? All the examples I see use some "key". I need simple things like (pseudo code):

user.Email = "new@email.com"
filter := string.Format("Id='{0}'", "4be0f045-6ce8-4d3e-751e-15bd84f2b925");
datastore.Put(context, "User", user, filter ), &user)

Also, probably related, how to get keys after I do

querySize := 1000
query := datastore.NewQuery("User").Limit(querySize)
queryResult := make([]User, 0, querySize)
query.GetAll(context, &queryResult)

"queryResult" contains all the records, what's their corresponding keys ? so I can do an update if I want to.

GetAll returns an array of keys and in case of an error an Error. Just iterate over the keys.
https://developers.google.com/appengine/docs/go/datastore/reference#Query.GetAll

You can delete a record with Delete instead of Put. Here is the App Engine Datastore reference for Go that explains all operations:
https://developers.google.com/appengine/docs/go/datastore/reference

And have a look at this example from Google. It shows how to get keys and update or delete records:
https://github.com/GoogleCloudPlatform/appengine-angular-gotodos/blob/master/gotodos.go

In order to update or delete a datastore entry, we need the key for the entry in question.

query := datastore.NewQuery("User").Filter("ID =", "4be0f045-6ce8-4d3e-751e-15bd84f2b925")
var u []User
key, err := query.GetAll(context, &u)

Now we have an array u with all entries that match the filter id = 4be..., in this case u has only one entry, and we have an array of keys, key, with the corresponding key for the entry.

To update the entry:

u[0].Email = "new.email@org"
datastore.Put(context,key[0],&u[0])

This assigns the first entry of u the new email and stores the changed entry in the datastore.

To delete the entry:

datastore.Delete(context,key[0])