You can easily change your login password at any time. This is most commonly done after using the password reset feature which generates a temporary password. (Password reset is done at the login screen and is not the same as changing your password)
Login to Pool Brain at dashboard.poolbrain.com
Click on your name in the top right corner of the page
Click on "Edit"
Click on "Change Password"
Fill in the fields and click "Update" - Your password is now changed