Windows: Highlight the text whose case you want to change and in the Home ribbon, click the All Caps icon:
Note that although the icon is called All Caps, if the selected text is in all caps, clicking this icon will convert the text to lower case.
Mac: Highlight the text whose case you want to change and on the keyboard press Command + K or go to Format > Toggle Case.
You can also add the command to the toolbar.
FD13 / 12 : Go to View > Customize Toolbar and drag the Toggle Case icon up to the toolbar:
FD11 / 10: Add the Styles icon; this icon set contains the All Caps icon.