In Mac OS X 10.4 Tiger, you can edit Address Book contact cards in a variety of ways, such as by clicking the Edit button or by choosing Edit Card from the Address Book Edit menu.
With Mac OS X 10.4 Tiger, you may have become accustomed to editing information in a card by double-clicking on a field. With Mac OS X 10.4.1 or later, you can still do this, but you'll need to hold down the Option key when you double-click.