The login/sign up button is at the top right (there is also a larger, orange button a bit below the header on right side for sign up).
Input your username and password and make sure to check "Stay logged in" in case you don't want to enter your password each time you start a new session.
You can log out of your account by hovering over your username and then clicking "Log Out".
Once your login id (or user id or display name) has been created at the time of registration, it cannot be changed by you. Please contact us if you wish to change.
Once your birthday has been entered, at the time of registration, it cannot be changed by you. Please 'contact us' if it is incorrect.