If you have forgoten your username, you can retrieve it by clicking on the log-in button and choosing Forgot your UserName? from the links at the bottom of the login page
As long as you can remember - and still have access to - the email address you used to create the account, your username will be sent there.
If you have forgotteen your password, it's a similar routine. Click on log in and choose 'Forgot your Password?'
Again, so long as you have the original email address, a password reset link will be sent to that address so you can reset the pasword. You can reset it to the same as before, up to 5 times, then you must choose a new one.
As you will probably guess, if you forget both username and password, you can still fix it - you just need to get your username, then use that to reset your password.
If you no longer have access to the email address you set up the account with, then you will need to contact the data administrator with proof of ownership, to have the email address changed.