How can I edit a file with .dll extension?

First things first: Android doesn't use DLL files, it uses .SO files. DLL's are for windows projects (or Xamarin possibly).

And editing a game by looking at its binary files is neither simple nor - in most cases - legal. It's a copyright violation and that's counted as malicious behaviour.
We do not condone, support, or assist in the production of malicious code in any way, form, or manner. This is a professional site for professional developers.

If you want to know how to do such things, you need to visit a hacking site: but be sure to disable all firewalls and antivirus products first or they won't trust you enough to tell you.