While updating a Nexus phone to Android 6.0 Marshmallow on Windows and Linux are trivial matters, things are a bit different for those who dwell in the land of the Macs. Thankfully there’s some help out there that should get you well on your way.
The video you see below was originally made for the Nexus 5’s Android 5.1.1 Lollipop, but thankfully it still fully applies to installing Android 6.0 Marshmallow on any modern Nexus device.
Otherwise, an attention to detail and the ability to sit through a 3 minute video are all you need to get yourself educated on what to do. Give it a go, but be careful. And remember, if you aren’t comfortable with manually flashing updates then there’s nothing wrong with waiting for it to hit your device naturally.