In Prezly's story editor, you can use the option in the formatting context menu. All you have to do is select the text which you want to format as subscript or superscript and click on the "S" button in the menu that pops up.
If you click it once, your text will be formatted as superscript.
If you click it again, it will be formatted as subscript.
You can click it one more time to format it as a regular text again.
If you want to see how to use subscript / superscript in the old Story Editor, please see this article.