Samsung Galaxy S6 and S6 Edge, the 2015 flagship devices from Samsung have now started receiving firmware update with August security patch. The update is being sent via OTA (over-the-air) to all Galaxy S6 and S6 Edge devices in several phases. The update  comes with build number G920TUES5EQH2 for Galaxy S6 and G925W8VLS5DQH1 for Galaxy S6 Edge respectively.
As stated earlier, this update brings in August security Patch, bug fixes along with improvements in stability and performance of both the devices. If you own any of the two devices, the update will soon be available on your device, if not already.
You should download the update as soon as it appears on your device as it is meant to improve the security of your device and addresses vulnerability issues.
Since the update is being sent over in different phases, however, you should get it anytime soon. You may also check the update manually by tapping Settings > About Device > Software Update.
It is worth noting that your device has sufficient power left before you start downloading the update. Also check that you are connected to a reliable Wi-Fi or Internet connection for the update to download seamlessly.
