# Credit goes to colewhitelaw.com for most of this solution.
# start an interactive shell-this assumes you are in the right directory
python manage.py shell

>>> from django.contrib.auth.models import User

# Grab all users and output
>>> userlist = User.objects.all()
>>> userlist
[< User:yourusername >]

# Refer to your required user by index and set new password
>>> userlist[0].set_password('newpasswd')

# Then save it
>>> userlist[0].save()