The default in LibreOffice is for slides to be landscape, but you can change slides to portrait (from landscape) in Impress easily.
- Open your presentation (or a new blank presentation) in Impress
- Click on Slide (on the menu bar)
- Choose Slide Properties
- The Slide Properties dialogue box will open
- Make sure you are on the slide tab
- In the top left-hand corner of the dialogue box, in the paper format section, choose Portrait
- Click OK
- Your slides are portrait 🙂
Change Orientation using the Side Pane
Note – If your side pane is not visible on your screen, click on View (on the menu bar) and choose Side Pane. This should put a tick next to it, and the side pane should appear on your screen.
You can also change the orientation of your slides using the sidebar in Impress. To do this, make sure your sidebar is visible (see above), then click on Properties. In the Slide section, use the drop-down arrow here to change the orientation for landscape to portrait.