extended warranty billing schedule update

Hi

Could you please let me know how to update the billing schedule for extended warranties which are sold from Order Management. Here is an example

AS54888 and WR 23763 ( just an example) was booked and shipped through order management . extended warranty is for one year and amount is XXX. both lines are closed and " Service contract Order processing request" is completed. Service contract is created and billing is enabled but there is no option to change the schedule like invocing should be done for 6 months . Please let me know whats the process involved to achieve this functionality

Regards