You can reset your password from within Angry Birds Go!. First, tap the account button at the bottom center of the game's title screen. Next, tap “Forgot your password?”, enter your email address, and tap “Send Request”. You will receive an email containing a link through which you can create a new password. Lastly, tap “Reset Password” and you’re done!
You can also reset your password at http://account.rovio.com.