This option can be found by going to Settings > Invoice Options >Field Names and here you have the ability to change 4 Fields, for example "Quantity".
Invoice Name - You can rename your invoices, to anything you like. For most people "Invoice" is fine, but some people may want to rename this. For example "Factura".
Credit Note Name - You can also rename Credit Notes if you want to. When you create an invoice for a negative amount, this is the title it'll have.
Customer Reference - You can include a customer reference on your invoices; this is often used for Purchase Order numbers. You can also include this field on-screen where your invoices are listed on the Sales page, to enable this just tick the option.
Qty - Rename the field to something more appropriate to you - perhaps 'Hours' or 'Days'. You can tick the box to completely remove the Quantity field from printed invoices, this is found under PDF themes.