Below are steps on how to move the navigation bar from the left side to the bottom:

1. On Outlook. Click File > Options > Advanced.

2. Uncheck the Show Apps in Outlook option under the Outlook panes.

3. Click the OK button to finish the set up.

4. A prompt box pops up, reminding you that you must restart this application for the changes to take effect. Click OK.

4. Restart Outlook. You will find the navigation bar is moved back to the bottom.