Select the text you want to format
Go to the Home tab
Click the Superscript button to make the text smaller and raised
Click the Subscript button to make the text smaller and lowered
Use the keyboard shortcut Ctrl + Shift + + for superscript
Use the keyboard shortcut Ctrl + = for subscript
On Mac, use Command + Shift + + for superscript
On Mac, use Command + = for subscript