See solution in other versions of Excel:
Question: How do I create a superscript value in a cell in Microsoft Excel 2003/XP/2000/97?
Answer: Select the text that you wish to convert to superscript. This can either be the entire cell or only a character in the cell.
While your mouse is over the selected text, right-click and then select "Format Cells" from the popup menu.
When the Format Cells window appears, select the Font tab. Check the Superscript checkbox.
Now when you return to your spreadsheet, you should see the selected text as a superscript value.