Whenever your customer likes your quote and decides to place an order, it's time to convert the quote into an invoice to make the sale!
Go to Income > Quotes, find the intended quote, click "Convert to invoice" in its Actions menu.
The data in the quote will be automatically brought to make a new invoice. The only difference is that you have to specify the Due Date in order to put it in the book and proceed to payments after, and Currency Rate if issuing the invoice in another currency.
Of course, you can add some changes to your new invoice if the customer wants to order differently.
For a detailed tutorial to issue an invoice, read here: