How to change the user display name to their first name in Django

For any such changes, you should refer to the template that's rendering it. Here, it is the Django Admin's base.html template.

As you can see in this line in the file, it searches for both short_name and user_name in that order and displays the first one available.

{% block welcome-msg %}
    {% translate 'Welcome,' %}
    <strong>
        {% firstof user.get_short_name user.get_username %}
    </strong>.
{% endblock %}

And get_short_name returns the first name of the user. So, your user does not have their first name defined and hence it's showing up their username.

NOTE : Please check your Django version's documentation since this has been implemented after version 1.5 and above, and is valid only for greater versions.

I highly recommend to extend the auth user model and thus you can do a lot of customizations sooner or later.

You can use lambda function to alter the __str__() method of auth user as

from django.contrib.auth.models import User

User.__str__ = lambda user_instance: user_instance.first_name

Note: This snippet should get executed on the Django server initialization

If you are trying to change the username which is shown at the top-right-corner (as mentioned by @amit sigh ), set the lambda function for get_short_name as,

from django.contrib.auth.models import User

User.get_short_name = lambda user_instance: f"Prefix : {user_instance.first_name} : Suffix"