Open your app and click "Log in". Click on the "Forgot your password?" text below the "Log in - button". 
Enter your Golf GameBook account email address and click "Send".
Or you can order reset mail from here.
Remember to check your emails spam box in case you can't see the mail in inbox.
If you are still having trouble with resetting your password, then let us know at support(a)golfgamebook.com and we will help you right away.