The TAB key cycles through all the commands in one group on the tab before moving on to the next group.
You can also navigate the Ribbon by using the TAB key. After you press ALT to move the focus to the Ribbon, pressing the TAB key cycles through all the commands on the active tab, group by group.
After cycling through the last group of commands on the tab, the TAB key moves the focus to the Help button, the Microsoft Office Button, and the Quick Access Toolbar, and then back to the first group on the tab.
Press SHIFT+TAB to cycle through the commands in the opposite direction.
When the focus is on the command that you want, press ENTER to select it.