You can create an invoice from a quote or contract. When editing a quote look for the "Quote" button on the top menu or open the “Date, Times and Invoicing" tab. There you will see the invoice button, click on it and enter the invoice date. Once done save or send the invoice. The quote is now an invoice.
IMPORTANT!! AN INVOICE IS NOT CONSIDERED A CONTRACT
IMPORTANT! Once an invoice number is assigned, you won’t be able to revert it back to a quote. If you want to void the invoice you will have to create the invoice to contract and then void the contract and invoice together.