Superscript on Mac Preview

In the Keyboard tab of Keyboard system preference, enable "Show keyboard and emoji viewers in menu bar".

Click the Keyboard viewer menu bar icon and select "Show Emoji and Symbols".

If Digits is not in the sidebar, click the gear near the top-left, select "Customize List", enable Digits, and click Done.

Select Digits in the sidebar.

The superscript digits are near the bottom. Double-click a digit to insert it in your text.

km²

I use Preview a lot and do not see a way. The text editor in Preview is rather simple-minded and offers only the most basic functions.

Excellent find Malcolm !

+1!