In the cell, select the character that you want to put in superscript or subscript.
If, for example, you want to write H20 with a subscript 2, select the 2 in the cell (not in the input line).
Open the context menu for the selected character and choose \<emph\>Character\</emph\>. You will see the \<emph\>Character\</emph\> dialog.
Click the \<emph\>Font Position\</emph\> tab.
Select the \<emph\>Subscript\</emph\> option and click OK.