In your projects' root urls.py file, simply add the below code to disable the new sidebar feature.
from django.contrib import admin
admin.autodiscover()
admin.site.enable_nav_sidebar = False
Reference:
It looks like one of two problems.
Your browser is caching the CSS / JS from the old version, in which case clear your cache and reload.
You didn't run ./manage.py collectstatic after updgrading.
Judging from your comment, it's probably the former.
Your browser is caching the CSS / JS from the old version, in which case clear your cache and for that, you need to do Force Reload.
@Tom Carrick, you are right btw.
and to Force Reload in Chrome use Hold the Ctrl key and press the Reload button. Ctrl + F5 also works
and for Mozilla Firefox Ctrl + Shift + R
For development/Production ie. on server just run
python3 manage.py collectstatic
This will bring all the html and css files for you and put that in static folder, that you have defined in settings.py.