1. Go to Control Panel, indexing Options

2. Inside that window, click on "Microsoft Outlook" and then click the "Advanced" button

3. In the middle of the screen on the next window, you will see "rebuild" button, click it.  Click "ok" when the warning pops up. 

4. Then close the window and the control panel.  It will take a few hours to re-index each user, depending on how big their mailbox is.