Navigate to the "Main" tab of the Project Configuration page (Project Menu > Set Up > Project Configuration) page of your project.
Expand the "Transmittal" section and select any Transmittal Templates that are available for use from the drop-downs. You can select different transmittals to be used for your Customer (shown as Acme in screenshot below) and Sub-Supplier.
Note: Internal/3rd Party Transmittals will use the selection you made on your Sub-Supplier Transmittal.
Once the desired updates have been made, remember to click Save Project to apply the change(s).
For more information on working with Templates please check out our video: Templates