Android Studio Project edit function

I have a project for an App like WhatsApp purchased on codecanyon.
The code uses Firebase and I need to modify some functions, example:
- I don't want the registration to take place by phone number, but to be free only with a name
- Minor changes to some functions, colors and logos

we have to use the App internally, so the most important change is the registration which doesn't have to be there.

The code I have is this: https://codecanyon.net/item/fireapp-chat-android-chatting-app-with-groups-inspired-by-whatsapp/22453192