I would like to send a newsletter that I've already sent with some alterations. Can I somehow edit this already sent newsletter?
You can make a delivery based on a sent message by clicking the newsletter you want on the Reports page. The row turns gray. After this click Open the newsletter for re-delivery on the toolbar and the message opens in the editor. The new draft has the previous delivery's settings and recipients by default but you can edit these in the delivery stage if you want to.
The latest sent messages can also be opened for editing on the Compose message page, on the Sent messages tab.