Unfortunately, Prezentor doesn’t have an undo-button yet. We're working on it!
However, you can use the "Save" button whenever you're happy with changes.
The Save button will only ensure that your current editing is up-to-date. The changes won't reflect in the finished version of the Presentation (in the App) until you press "Publish."
Therefore, while editing, instead of using an "Undo" button, you can use the "Cancel" button.
If you click cancel you cancel all changes since last saved.
Therefore, it is important to regularly save your changes while making a presentation, when you're happy with the content you've created so far.