You can edit the Client's invoice frequency through the Edit Client overlay. On a Client Account, click Edit next to the Client name to open the overlay. Look for the When To Invoice field on the Billing tab:
You also can bulk-edit the invoice frequency on the Client list: CRM > Clients > Actions > Bulk Edit. At the When to Invoice field, select an option from the dropdown list. This update will apply to all Clients you have selected from the list.
If you update the Invoice Frequency at the Client level, this change will populate down to any existing Master Jobs as well as any Job instances that are Pending Dispatch. This change will not affect any Jobs that are currently Dispatched or Complete.