2. Under Actions menu, Select "Edit Google accounts info" for the Adwords user credentials used in Marketo
3. That will bring up a separate page - myaccount.google.com.
4. In the "Sign-in & security" page there's a "Connected apps & sites" section that lists all of the apps connected to the account - one of them should be Marketo.
5. Clicking the "Manage Apps" link should bring up a list of apps, and clicking on the Marketo app should give you the option to Remove.
6. Once removed, you should be able to reauthorize the account in Marketo.