When you open the Invoices page, first of all, you need to use the Search pane to find the invoices that you want to manage.

If you want to manage one invoice, you can use options in the columns... 

...or click the Select button in the Action column and select the option that you want. 

If you want to manage one or multiple invoices, you can select the check box next to the appropriate invoices, and then click one of the options on the toolbar.