How can I change the username and the password of a user in django programatically?

I am creating an application in django and I have a user who can change the passwords and usernames of the other users.

In my views.py I have a view that controls this.

I get the user of a concrete username this way:

user = User.objects.get(username=user_name)
user.username = request.POST['username']

user.set_password(request.POST['password'])
user.save()

But when I save all, I see that the selected user's username didn´t changed.

How can I set the username and the password of the selected user "user_name"?

Thank you!