[Shield Android TV Themes and Apps] DBCAB:
OK this has been covered in these threads

http://xdaforums.com/nexus-player/general/how-to-set-default-launcher-marshmallow-t3227812

https://forums.geforce.com/default/topic/929643/auto-start-spmc-/?offset=5

But I figure post here with clear directions in case anyone needs, ty Asadullah and FantasticN

First find a launcher you intend to use I use The Ha launcher it can be found here

https://play.google.com/store/apps/details?id=net.i.akihiro.halauncher&hl=en_US

and install (It can also be found on the play store on the Shield)

On Shield TV

Marshmallow

Activate Developer mode: got to settings-Info and press build 8-times

Then go to Settings--Preferences--Developer Options and activate debugging for USB

On your PC (if needed) download Windows USB-Drivers for Shield TV and install:

https://developer.nvidia.com/shield-open-source

Download Minimal ADB and Fastboot and install:

https://xdaforums.com/showthread.php?t=2317790

Connect your Shield TV via USB to mini USB with PC, For the 2017 Shield Model please look below for connection info.

Leave the Shield plugged in and turned on as the way u usually use it connected to tv open command prompt (Minimal ADB and Fastboot ) on PC and type adb devices the PC will try to connect with Shield TV via USB. A message will pop up on the tv requesting permission for that device. Confirm that message with the controller or other device you use mouse, remote etc.

Type in the command prompt

adb shell

pm hide com.google.android.leanbacklauncher

To undo this

adb shell

pm unhide com.google.android.leanbacklauncher

This will make the new launcher the default as it hides the leanback

(Commands are different for the Nougat build see Update below)

Also If this is needed because you cannot get into settings on your device afterwards I have attached a copy of the shields setting apk file that I got off the nvidia site just install it as you would any other apk and all is fine access all settings etc.

This should work fine with any launcher that is setup as a Home Launcher please make sure you have a launcher that works and is what u need/want and that it is indeed set up and set as Home launcher.

UPDATE : For Shields running Nougat build

This is for Nougat NON Rooted shields, This also works on the 2015 model.

For Nvidia Shield 2017 Model Please follow the updated instructions After you have set up minimal adb and fast boot on PC.

First get a Type-A to Type-A USB cable or an adapter that makes micro usb to Type-A

Use the Usb port furthest away from the Hdmi Port On The Shield TV

Select Usb device mode In Settings

Select Settings--Storage and Reset -- Using Usb (for Mtp)

Select Settings--About--Build-- Press Build 8 times to enable Developer mode then

Select Settings--Developer Options--Usb debugging (For adb)

Then

Type in the command prompt:

adb shell

pm uninstall -k --user 0 com.google.android.leanbacklauncher

This should work fine with any launcher that is setup as a Home Launcher please make sure you have a launcher that works and is what u need/want and that it is indeed set up and set as Home launcher.

UPDATE: OREO Here are the new commands for the Oreo update

Type in the command prompt:

adb shell

pm uninstall -k --user 0 com.google.android.leanbacklauncher

and

pm uninstall -k --user 0 com.google.android.tvlauncher

This will uninstall the Default launcher on non-rooted shields please make sure you have a functional launcher, set as Home launcher before doing this.

UPDATE: Pie (8-3-2019) To enable adb debugging on Pie go to Settings -- device preferences -- about then scroll down to build select and click it until developer mode is enabled.

To Disable the New Pie Launcher use this command

adb shell

pm disable-user --user 0 com.google.android.tvlauncher