These actions can be taken in the presentation editor. From your Documents dashboard, just click on the presentation and then hit Edit.
To reorder slides, use the sidebar on the left-hand side and just drag and drop slides to the desired position in the deck.
Still in this sidebar, you'll find each slide thumbnail has a little arrow at its side. Click on it to get options like Delete and Duplicate. Please be mindful when deleting a slide, since this action can't be undone 🚫
You'll also be able to Insert new slides, before and after the selected one, as well as to Import slides from different presentations into the current one.