How can I install Marshmallow? Apparently there's this really fun flappy bird game built into Marshmallow that's even better than the one in Lollipop. I've got a Nexus 5 and really want to play it ASAP. How do I go about doing this?



I'm looking for both a Multirom-compatible zip and the conventional way which involves wiping your device.

How can I upgrade my Nexus 5 to the latest and greatest version of Android?
Conventional Way
This is what you want if you have a Nexus device but you've never heard of/don't use MultiROM. You should have an unlocked bootloader. If you don't have this, take a look at the useful CM guide to unlock your bootloader.
Also, make sure you have adb and fastboot installed and know how to use it. Backup any and all data beforehand (doing a nandroid backup is probably best)

Grab the Marshmallow factory image from here. The Nexus 5's codename is hammerhead and you're looking for the 6.0.0 (MRA58K) file.
Check the checksum of your downloaded file against the one listed on the page just to be safe. This ensures the download went smoothly and you don't accidentally flash corrupted version onto your device
Untar the downloaded file, extracting recovery.img, bootloader.img, system.img, etc.
Connect your phone to your computer via the microusb cable. Use adb devices to make sure  adb is working fine (turn Developer mode and USB debugging on first).
Use adb reboot bootloader to enter fastboot mode
In Linux/OSX, cd to the folder containing the extracted files and run sh ./flash-all.sh to flash everything. If you're running Windows, double-click the flash-all.bat file.
Your device should automatically reboot to your new Marshmallow system. (Note that the first boot might be agonisingly long)

MultiROM
Here's what you want if you use MultiROM. This method has the benefit of allowing you to keep your old system and all your existing data. You can then easily switch between Marshmallow and whatever you were running previously.
You should have an unlocked bootloader and a custom recovery like TWRP installed, of course.

Grab the MultiROM-flashable zip from this XDA thread. (You may also want to grab and flash the radio.img if you didn't install the Developer Previews.)
Reboot to your custom recovery and add a new ROM in the MultiROM menu.
Use adb sideload hammerhead_mra58k.zip (or adb push beforehand) to transfer the zip file to your device
After installation, reboot and choose the Marshmallow ROM to boot into