[Nexus 6 Q&A, Help & Troubleshooting] qnc:
if you have drivers installed already from first link above then you can follow these steps:

Download the required factory image files to your computer and extract them

1) Create an empty directory. All of the files that are required, need to be in one (i.e., the same) directory. It does not matter what it is called, but all the files need to be in there, with NO subfolders. EVERYTHING needs to be extracted into this one directory; (it is easier this way)

2) If all you want to do is unlock your bootloader, skip to step 6 in this section.

3) Download the version of Android you want to your new directory. Make sure you are using a nexus 6 image.

4) Extract the files from within the .tgz file which you downloaded in step 3 using 7-zip, not WinZip or WinRAR or whatever.

5) Make sure you have extracted ALL the files (including extracting any files in any archives inside the .tgz file). You should have six (6) files ending with .img in the directory you created in step 1. The other files you extracted from the .tgz are not necessary;

6) Reboot your device into bootloader mode (by turning it off, hold volume down, and press and hold power) and plug it into your computer;

7) Open a command prompt in the same directory (i.e., make sure you are in the same directory as your files are located). You can hold the shift key when you are in the folder in Windows explorer and right-click in a blank spot and it will open a command prompt. In Ubuntu just cd to the directory.

Type in the commands into the command prompt

1) Make sure your computer recognizes your device by typing: fastboot devices

2) Unlock your bootloader (if you have not already done so): fastboot oem unlock

3) You will see a prompt on your device. This will wipe your entire device (including the /sdcard folder). Accept. Note: you use the volume keys to change the option and the power button to accept. You cannot use the touch screen.

4) Reboot by typing: fastboot reboot-bootloader

5) Flash the bootloader: fastboot flash bootloader bootloader-some-description-number.img (or whatever the name of the bootloader image that you downloaded).

6) Reboot: fastboot reboot-bootloader

7) Flash the radio: fastboot flash radio radio-some-description-number.img (or whatever the name is of the radio image that you downloaded).

8) Reboot: fastboot reboot-bootloader

9) Flash the system partition: fastboot flash system system.img

10) Optional, but read note. Flash the data partition: fastboot flash userdata userdata.img Note: this command will wipe your device (including /sdcard), EVEN if your bootloader is already unlocked.

11) Flash the kernel/ramdisk: fastboot flash boot boot.img

12) Flash the recovery partition: fastboot flash recovery recovery.img

13) Erase the cache partition: fastboot format cache

14) Reboot: fastboot reboot

15) Done! The first boot will likely take quite a bit longer than you are used to, as Android builds the cache.

Now you have a fully stock image on your device. The only thing that is different from out-of-the-box condition is that your bootloader is unlocked.

No you can root or decrypt. Search for directions on the forum or the internet.

I'm not the original writer of these instructions, but I found these helpful to people wanting the step by step instructions.