In Office’s built-in equation editor, you can type ^
(caret) for superscript followed by your text, and subscript is _
(underscore) followed by the text.
You must enter a space after entering. For example,
e_r SPACE next letter
will give you
er
(Reprinted by https://superuser.com/questions/128240/how-to-write-subscripts-in-microsoft-office-formulas)