You can manually upgrade or use the automatic option to upgrade to the latest firmware version. The firmware can be downloaded from the WD Support website.
It is possible to downgrade but it generally takes a little work via SSH to do so.
See the following link for more info on various firmware and how to downgrade. Note you cannot upgrade an older v4.x to newer v2.x same for v2.x, you cannot upgrade newer v2.x to older v4.x.
