@johnnyboy - In the latest version of Raspbian Buster, you can rotate it through the UI. But you have to do a full OS update so as to include the latest version of the desktop.
On the pi, open a terminal window (ctrl+t) and update everything: sudo apt update && sudo apt upgrade -y
That step will take a while to complete. Depending on your model of Pi, expect 30 minutes or so. It will require a reboot at the end, so reboot your Pi.
Once the Pi is back up, you’ll be at the desktop. If you’ve already installed Magic Mirror, you’ll want to stop it by opening a terminal window and entering pm2 stop all. Go to the Raspberry button follow Raspberry -> Preferences -> Screen Configuration. The configuration dialog will appear. Follow the menu like so: Configure -> Screens -> HDMI1 -> Orientation and make your selection to rotate the screen. It will enable the green check mark which will apply your selection.
Select whichever orientation makes sense for you. Just so you know, the descriptions describe the bottom of the monitor. So the “left” entry puts the bottom of the monitor on the left in portrait mode.