Marshmallow’s rollout is so far typically slow, but Nexus 5, 6, 7, 9 and Player owners can upgrade manually. Before you get started, make sure that you’ve backed up your device and that you’ve got all of the necessary kit and software tools. You’ll find the tools you need and instructions on how to use them in the links below. Follow the instructions carefully!
Download the stock ROM for your device. Extract the contents to C:Program Files (x86)Minimal ADB and Fastboot. Open Developer Options and enable OEM Unlocking and USB debugging.
With the USB driver installed and with a back up, run Minimal ADB and Fastboot. Enter adb reboot-bootloader in the prompt then (on a locked handset) follow with fastboot oem unlock.
Now install Marshmallow. Enter flash-all in the command line and wait. When restarting, you’ll see the new version. As it installs as an update, previous apps and settings are maintained.