To view an Invoice, you need to click Invoices tab in the left menu that will take you screen with all the invoices listed. You can choose to view any Invoice by clicking the 'View Invoice' button in action buttons section. This will take you to the Invoice Details page. This page has three sections, Summary, Comments and History.
In the Summary Section, you can see Invoice details, payments and payment statuses etc. In the comments section, you can view all the comments left by customer on Invoices and replies by you. And in the History Section, you will find all the activities recorded regarding that Invoice.
On this screen, you can click 'Edit' button and it will take you to Invoice Editor Screen, and there you can edit any detail you like. You can also choose to add or delete items to Invoice. Right next to the 'Edit' button, you will find 'Mark Paid' button that you can use to record payment for Invoice.