A PreBill invoice is a draft invoice. It is editable, and you can click right on the time and expense entries in the PreBill Invoice. The original Time Entry and Expense Entry windows will pop-up, where you can make any edits. When you click "Save" the PreBill Invoice automatically updates with your changes.
All invoices pass through the PreBill stage before you finalize and send them to Clients for payment. In order to send an Invoice to a client, you should click the "Finalize" button on the Invoice. Finalized invoices are locked.
You can search for and see all PreBill and Finalized Invoices on the "Billing" Tab, "Invoices" Sub Tab.