How to change default camera?

Hay guys,

i could not find forum for lg joy, so, i postED here.

I have a question about camera.
Lg joy has option in settings to start camera app, when device is locked, and when screen is ofF by long pressing volume down button.

This turns oN the screen and starts the camera.

What i want to do is to change default camera app to non stock, so, that when i long press volume down when my device is locked, that other camera app start, witch is not the stock one.

So, do you know how to do this?

I already tried going in settings►applications►all►camera and clearing preferences, but that button is disabled, saying that there is no preferences.

So, anything else i can try?

I just want to be able to start camera on a wery fast way, and without display going on.
Since, i kind of love spying, i have an app, wich can run in background, and i can take pictures without turning on my screen, and without unlocking it.
The only time when i need to unlock the phone is when i am starting this app.
But since, i kind of like spying, and usually, want to be able to take picture in a few seconds, after i notice something interesthing, i sometimes do not have time to turn on the screen and to launch the app.
And stock camera is not good because it requires screen to be on, and i want to be undetected.
So i need a way to launch camera app, but in the background, within 5 seconds.
That is why i want to change default camera witch is started by long pressing volume down button.