Once you issue an Estimate or a Quotation, you can easily turn into an invoice by going to the Dashboard and then then following these steps:
1. Go to the Quotations tab, by clicking the "Quotations" link on the Dashboard, located under Reports.
2.Here you see a list with all issued documents of this type. As per the screenshot to the right, all you have to do is click on the icon in the corresponding "Action" menu - the icon with a "+".
3.The invoice editor will be opened with the quotation data preset.
Make any modifications you want and then all you have to do is click "Preview Invoice" and then "Save".