I understand your eagerness for the latest software update, tit4tat. At this time the available software for Samsung Galaxy S5 is available to download over the air through the device settings. Marshmallow has not been made available. You can check what updates are available for devices here: http://spr.ly/6586BwfAa
Follow us on Twitter @VZWSupport
If my response answered your question please click the "Correct Answer" button under my response. This ensures others can benefit from our conversation. Thanks in advance for your help with this!!