How do you show a number to the second power on a computer?

You can either use superscript - a typographical device which raises the characters off the base-line and reduces their size.... 52 ... or you can use the caret mark, the programming sign for powers, 5^2, but that is not always recognised by people who do not know computers.

You might be best advised to just write 5 squared, 5 x 5, or 5 * 5