Currently, only Nexus 5X and Nexus 6P are pre-installed with Android Marshmallow 6.0. Unless you need the specific version of Android for some of your apps, or some configurations, you should update your device to Android Marshmallow 6.0 update. Of course, in the history, some vendors may mess up the update because of some stupid customization. Please, there is no official way to roll back in Android devices.
All other released Android devices have to get Android Marshmallow 6.0 update through firmware update (aka system update). There is no perfect software, although some software is good enough for certain usages You should get Android Marshmallow 6.0 update for your device because of: Some “experienced” Android users may tell you not to update because of “new” bugs. But this happens less and less frequently in Kit Kat and Lollipop update. In other words, if you apply the Android Marshmallow 6.0 update update for your device, you cannot easily roll back to Lollipop.
So, even if the vendor (e.g., Samsung) pushed out Android Marshmallow update to the carrier for testing, you still need wait from 1 month to 6 months (yes, some carriers are really slow in approving update) to get the Android Marshmallow update.
It is not uncommon that in the same country, some carriers release the Android Marshmallow update earlier than others.
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For any carrier specific models, the updated firmware must be approved by individual carrier.
They may do some tests to make sure the firmware works with their network.The Android Marshmallow 6.0 update you get through PC software usually is identical to that through OTA. Please note, all these software tools are proprietary and work on their own devices only.For example, Samrt Switch PC software only works for Samsung devices, not on LG devices. Please note, when flashing the factory image, you will remove all your data, settings and apps.You can use them to Android Marshmallow 6.0 update for your device from a PC.Please note, you need connect your device to PC with the software installed through a USB cable. Some software checks update automatically, for example, Smart Switch.Android Marshmallow 6.0 update are to be rolled out by individual vendors.