If you are logged in to your account, you can generate a set of backup codes on your Security tab here:
Please be sure to turn off any pop-up blockers in your browser. You will need to use one of these codes to disable two-step authentication on your account. You can then re-enable it and set it up with the newly installed Google Authenticator App.
You can find more information about two-step authentication here:
Two Step Authentication
If you have lost access to your account, please use this form to request assistance regaining access: