How to update() a single model instance retrieved by get() on Django ORM?

With the advent of Django 1.7, there is now a new update_or_create QuerySet method, which should do exactly what you want. Just be careful of potential race conditions if uniqueness is not enforced at the database level.

Example from the documentation:

obj, created = Person.objects.update_or_create(
    first_name="John", last_name="Lennon",
    defaults={'first_name': 'Bob'},
)

The update_or_create method tries to fetch an object from database
based on the given kwargs. If a match is found, it updates the
fields passed in the defaults dictionary.

Pre-Django 1.7:

Change the model field values as appropriate, then call .save() to persist the changes:

try:
    obj = Model.objects.get(field=value)
    obj.field = new_value
    obj.save()
except Model.DoesNotExist:
    obj = Model.objects.create(field=new_value)
# do something else with obj if need be