Hello,
I am Ravindra Barthwal, a Udacity certified android developer. I have experience in dealing the projects of these kinds. I have got trained by leading industry experts from Udacity and Google (through their Nano degree)
As you said you want a tutorial on updating access key of firebase on your app. So that you can have a second app that will have different firebase accounts with all the app functionality.
I am an experienced dev and have done this kind of project, In fact, the last app I created for my client used firebase and I use two firebase account one of his and second is mine(for demo) and we used to switch between each other easily.
I can help you with this.
Regards