To issue new invoice go to Invoices > click the Issue New Invoices button ( upper right corner). Choose the filters you need: Issue invoice for, From date / To date, Customer and click Calculate invoice button. Select the reference you want to bill than click Issue.
​If you want  to change dates such as invoice date, due date, service description, add new service line or discount, timesheets, then simply Edit the invoice. Select the Save as fiscal button. You can validate the invoice from Actions > Validate.
If you want to issue a proforma, select Save as proforma. You can create more than one fiscal invoice from a proforma from the Actions button > select Force allow issuing a new fiscal.