To change the default settings such as friendly fire you will need to generate a world locally on your game and then save it.

Then stop your server and delete the existing world (the contents of the "Bedrock level" folder).
Connect via FTP and upload your locally created world. On windows you will find it in the folder:
C:\Users\YOUR WINDOWS USER NAME\AppData\Local\Packages\Microsoft.MinecraftUWP_8wekyb3d8bbwe\LocalState\games\com.mojang\minecraftWorlds\

On android phone the files are in Phone> Games> com.mojang> minecraftworlds. In this case you can either use an FTP client for android or transfer your files to a PC before sending them from the PC to the server by FTP.

Start the server.