Can someone help with IOS game ap?

I was reading a thread on here about finding an app source code and editing it a bit to make it unique. Well I found the DOOM one and I have some good ideas but I have no clue how to edit it or test it.

I dont even see the images I am supposed to have.

If anyone can spare a few minutes of your time and are familiar with this I would appreciate it. I am wanting and ready to take action, I am just lost on this. I have searched all around but nothing I found is helpful.

Thanks