Go to:
/edx/app/edxapp/edx-platform
Then:
sudo -u www-data /edx/bin/python.edxapp ./manage.py lms --settings aws shell
from django.contrib.auth.models import User
me = User.objects.get(username="user_name_of_the_user_you_want_to_be_super_user")
me.is_superuser = True
me.is_staff = True
me.save()
(Edit: forgot the command to get into the shell)