For those of you who have the excellent LG G3 on the T-Mobile network, today is a great day. The carrier has started pushing out the Android Marshmallow update to the device, bringing it up to version 6.0 of Android and all the goodness that the release has in it. The updated version you are looking for is build H63120b and while T-Mobile on their support page do not given an indication of size, if other updates are any indication, you can expect about a 1GB download from Lollipop to Marshmallow. That means you will need to be on Wi-Fi to download it and you can expect the total update time to take about 30 minutes.
While the news has not come directly from Verizon, multiple sources are reporting that users of the LG G3 on the US-based carrier are starting to see Android Marshmallow come to their devices via an OTA update. The original source of the news came from Reddit and the original poster included a screen image of the update on their device. If this is legitimate, this is really good news on a lot of different levels. First, it is reasonably fast for a carrier to get the update out. It’s not great but it is certainly better than Verizon has done in the past on updates.
Second, and perhaps equally as important, the update appears to be Android Marshmallow build MRA58K which is the January 2016 update. That means that this update to the LG G3 is going have the latest security updates from Google.