How to change django wagtail's admin logo

Wagtail already provide the solution in the official documentation using django-overextends:

To replace the default logo, create a template file your_app/templates/wagtailadmin/base.html that overrides the block branding_logo as follow:

{% overextends "wagtailadmin/base.html" %}

{% block branding_logo %}
    <img src="{{ STATIC_URL }}images/custom-logo.svg" alt="Custom Project" width="80" />
{% endblock %}

Check Wagtail Custom branding for more details.

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(Edit Dec - 2020)

Note: In the latests versions of Wagtail django-overextends is not needed anymore. It uses now the default extends tag of Django templates. Consult the docs for more information

The logo is defined here:

https://github.com/torchbox/wagtail/blob/master/wagtail/wagtailadmin/templates/wagtailadmin/base.html#L7

To override it, you'll need an app which contains templates/wagtailadmin/base.html and precedes wagtail in INSTALLED_APPS.

Good luck!