Step 1. Write Unit Test Cases for the business logic. Business rules belong in the model -- mostly -- and you need to rework the model. Some business rules aren't really "business" rules and bleed through into control and sometimes even view. Folks often overstate what's part of the business and what's part of the current business process as performed through the current software.
Step 2. Fix the Model to have the correct business logic. Run the tests. Be sure things really work. You'll often find that you have to rework your model (and sometimes your tests) as you fix this.
Some rules will be in two places: their legacy places and the model. That's okay, temporarily.
Step 3. Start teasing the business logic out of the view/presentation/control layers. This will be hard, and will cause you to rework test cases and model. Rework is good, it's how you learn.
This can get difficult if you've buried a large number of assumptions in the presentation.