Sequelize.js how to use migrations

I understand how to write a migration file, however I'm a bit confused as what to do with the existing model definitions? Should those be updated so that they represent the latest version?

As in if I want to add a column to a table, do I create a migration that adds the column and update the model definition, or just leave the model definition alone?

Also how do I actually run the migration from a node.js app? is it possible to update the database using the migrations from code rather than using the CLI?