Changing the method of logging in
Right now I use google to login into Duo. I would like to be able to change that to using a Duo account and not google.
Is there a way to do that without messing up all my stats?
If you want to disconnect your google account, just scroll over your username and go to Settings>Profile>Google+ Connect> then click disconnect. Then you should only be able to log on with your Duo name and password and not your google account. This will not affect your stats at all. Hope this helps!
If you created the account using Google login, you may not have set a password for the Duolingo account. First check if there's an email address linked to the account in your settings page.
If not, you can try adding a valid email address and then try logging in using the email address/username and the password. If it doesn't work, you may need to follow the forget password path to generate a new password for the account.