1. In Records, Select Products and Billing Items 
  2. Open the Billing Item
  3. Select the GL Distribution tab 
  4. Edit the distribution 
  5. Save and Close the Record

This can also be done through Configuration

  1. In Configuration select Products and Billing Items
  2. Select the appropriate Item
  3. Select the GL Distribution Tab
  4. Edit the Distribution
  5. Click Save and Close