Afaik, you can install the WP8.1 version of Lumia Camera but cannot use it on W10M. I had very recently installed it on my phone being fed up of Windows Camera's poor quality. But the app simply had a good bye message and a button to open the built in Camera app. So, I think there's no way to have Lumia Camera on W10M, unfortunately.
do a hard reset first on w10,the Lumia camera will work,at least that is what i am thinkingI'm already on w10 mobile.. So i have to go back to 8.1, then update to W10 and leave lumia camera as it is,right?