Pull from Heroku to my computer (mac)

Pull from Heroku to my computer (mac)

I am very new to Rails, Heroku, Github and all the other stuff. My problem
is that locally on my Mac and unfortunately on Github i do have a "bad"
version of the application. Luckily i didn´t push it to Heroku. So on
Heroku i do have a good version of my application. Since i spend days
trying to find a bug in my application and got very upset, i want to go
the easy way and download the Heroku version to my Mac and replace the bad
one. And afterwards push it to Github. So that somehow possible? Already
tried $ git pull heroku master and got this error Your key with
fingerprint .... is not authorized to access myapp.
Getting really upset here. Haha. Thanks for the help in advance.