This has been tested on eRobot V6 which is based on Stock TouchWiz Marshmallow 6.0.1

You need root access.

Navigate to /data/system/users/0

Edit settings_system.xml



Look for

name="double_tab_launch" value="1" package="com.vlingo.midas"

change to

name="double_tab_launch" value="1" package="com.sec.android.app.camera"



If you are using Lollipop, editing SQL database in /data/data/com.android.providers.settings/databases/settings.db should also work.

PS. This is better method than my previous version of this original post. In previous method, you need to turn on phone screen first, then double home to camera will work. Which is triple home to launch camera behind lock screen. So you have to swipe to unlock lock screen first.

This new method, if phone screen is off, and you double home, it will launch camera right away behind lock screen. So you have to swipe to unlock lock screen first.