If Billing Schedules have not yet been generated, correct the amount on the billing item:
  1. In Configuration, click Products and Billing Items.
  2. Open the Billing Item.
  3. Update the amounts.
  4. Save and Close the billing item. 
  5. Run Generate Transactions Automatically generate the billing schedules.
To correct Billing Schedules that have already been generated but have not yet had charges generated:
  1. Globally delete the billing schedules.
  2. Complete steps 1-5 from above.