You can mod it in a computer and phone.
1)Computer- You can convert its apk to .zip and unzip its files and change them.
2)Phone- You can use android apps to mod them. There are many of them like apk editor, android studio etc.
go to settings and find the option of view file extension if you're on win 7 or earlier. If you're at win 10 then go to any folder and click on view tab and enable view file extension. Then change the file extension of the apk file from .apk to .zip or .rar. Then use any unzipping software like WinZip etc to unzip it. Then you will be able to see all of its files and modify them. Then after that change the extension from .rar to .apk again and install on it your phone.