The update will load the software version LPE23.32-21 for the Moto X and the U.S. Cellular version of the Moto E LTE has also started receiving the Android 5.1 update with verion LPI23.29-15.
Android 5.1 will brings tons of new features like new icons to Quick Settings and support for HD calls and dual SIM capabilities are also a part of the update.
The smartphones will get an important security feature with this update, which locks your lost or stolen handset until you sign in with your Google user name and password even if the smartphone gets factory reset.
The update is rolled out on a staggered basis and it might take few days to get update on some devices. Once the update is available to your device, your handset will receive notification to update to Android 5.1.
Keep in mind, both smartphone need to be connected to a Wi-Fi network to download and install the update file.