When you add a student for the first time, one of the required steps is to choose a billing or payment method. You can choose to bill them per session or a flat fee for a period of time that you can adjust.
If you ever want to change a student's billing method, follow these steps: Go to Students->Student List->Click on the row of the appropriate student. In the section titled "Billing Method" enter the new cost in the cost field and then choose between "Per Session" and "Flat Fee". If you select flat fee, some options will appear that you can adjust.
After your changes have been made the click submit and the student will begin being billed according to the new settings.