To update one of your models using South, you'll first want to create a new schema migration by using the schemamigration command and the -auto flag.

$ ./manage.py schemamigration some_schema --auto

if the model or fields were updated since your last migration, you should see something like this

 + Added model some_schema.ArticleCategory
 + Added field category on some_schema.Article
 Created 

Next, when you're ready to apply the update you can run

$ ./manage.py migrate some_schema

Check out the official South tutorial for more details.