Tested with django 1.9
from django.db.models import F, Func, Value
ExampleModel.objects.filter(<condition>).update(
string_field=Func(
F('string_field'),
Value('old text'), Value('new text'),
function='replace',
)
)
UPDATE Django 2.1 https://docs.djangoproject.com/en/2.2/ref/models/database-functions/#replace
from django.db.models import Value
from django.db.models.functions import Replace
ExampleModel.objects.filter(<condition>).update(
string_field=Replace('string_field', Value('old text'), Value('new text'))
)