Go to /forgot-password, enter your account email, and click Send reset link. We always return success even if the email doesn't have an account, to avoid letting attackers enumerate users.
The reset link arrives within a couple of minutes and is good for one hour. If you don't get it, check spam and the Promotions tab in Gmail.
If you signed up with Google originally, there is no password to reset — go straight to /login and click Continue with Google.