You can edit the Contact details without leaving the Scheduler screen by clicking on the job in the Scheduler > View Client Details.

This will open up a slider drawer on the right side of the screen, where you can then select the Edit pen at the top to edit the Billing Details, or select the Contacts tab, and then the Edit pen of the contact that you would like to edit.

Make your changes, and then Save and click off the slider in order to close it and return to the Scheduler.