As mentioned everything was working fine, but then I tried mirror display and the scaling went bad. Now I can't access the setting to undo it because it's at the bottom of the window and won't fit in the screen.
I just installed elementary OS and everything was working, but now display scaling is messed up
I had the same problem setting Scalling factor: to Pixel Doubled and the only way I found to resolve this is by 'guessing' pressing Tab and Enter again and again until i saw the sacalling factor select on the screen and I choose again LoDPI. If you have external monitor connected and you didn't messup with the system shortcuts, you can achieve this by Super+p which is used to circle between known display modes.
Just thinking off the top of my head here: Can you click on the title bar and move the window towards the lower right-hand corner just a bit? If you can, you should then be able to get a screen resize arrow. Then try re-sizing the window to where it's small enough and you move the window so you can access the settings.
Hold ⌘ (super key) and click and drag on the window to move it. This way you can get to the bottom of the window.