The Sync button has been moved to a new location! Here are the steps to access it.
  1. From the Home screen, click on the Gear
  2. Select Site Contents from the dropdown menu
  3. Click on the Documents library tile
  4. Click on the Sync icon
  5. Click on Sync now
  6. Click on Show my files…
  7. Click on the SharePoint folder, then your store number Documents folder.
This information is also available on the Store Support Website Doc#723