Software update is sent out to Motorola Moto Z users to fix low notification volume issue and more

Keep in mind that this update is for the unlocked version of the Moto Z, not the Verizon exclusive Moto Z Droid sold in the U.S. And because it is being sent OTA, the update is being disseminated in waves. That means it could take a few days before it arrives. You can try to manually download the update by going to Settings > System updates > Check for system update.
Once you receive notification that the update has arrived, make sure that you are connected to a Wi-Fi signal before starting the updating process. Also, be sure to have your battery charged fully before you install the new files.
source: AndroidAuthority