If you'd like to edit a scheduled invoice, go to Billing ---> Invoicing ---> Scheduled Invoices. Then click on the invoice schedule you'd like to edit. When you schedule a template, the invoices will automatically be sent on the 1st day of the month. If you'd like to change that, then click on the scheduled invoice to edit it.
When the scheduled invoice is active, the preview sidebar will be green; when it is inactive, the preview sidebar will be red.