Log in to your Google account or log in to your gmail.
If you log in through gmail at the upper right hand corner click your name/image then select account.
If you want to just undo a Google plus, under account select edit profile. In your profile click +1s from here you can delete your +1s.
If you want to disable or get rid of plus follow these steps: On the left hand side, select Google + and click edit. Next, select disable or scroll to the bottom and delete your google plus account.