With MS Excel you can superscript a whole cell but not individual characters unlike Word. Here is a way to denote a square or cube of a number using the Find and Replace dialog box.
1) From the Home tab, select Find and Select and click Replace. Alternately you can press Ctrl + H to open the dialog box.
2) Enter the number that you want to superscript in the Find What box.
3) In the replace with box enter the base number and then by keeping the ALT key pressed, press 0 + 1 + 7 + 8 one by one to add the superscipt 2 and ALT and 0 + 1 + 7 + 9 for adding the superscript 3.
4) Click the Replace button and you are done.
This tip is for a cell with only numbers. If a cell consists of alphabets and numbers, you can easily superscript a number by first highlighting it and then by right clicking > Selecting Format cells and checking Superscript check box.

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How about superscripting other numbers like 3,4,5 etc
You can add the superscript 3 as shown in 3rd step but I havent found a way to add other numbers yet.