If you've forgotten your username for logging in then you are able to go to "Username reminder" in the "Help Me" section above and enter the email address associated with the account - you'll then receive a reminder of your username.
To change your password (once you are logged in) is much more straightforward. At the top right hand corner of the screen you will see your avatar followed by your login name.Next to this is a tiny cogwheel and if you click on it you will get a little drop-down menu that includes an option to "Edit Profile". When you click on this option you'll be taken to the profile information you have recorded on the system. Just under where it says "Edit Profile" (now in the main body of the screen) are two items. These are "Information" and "Account". If you click on "Account" it will display your Login name, your email address and two empty boxes that allow you to reset and verify a new password. If you enter your chosen password in each of these boxes and then click "Save Changes" your password will be reset to your chosen value. If you leave them blank and just leave the page (WITHOUT clicking Save Changes) then the password will stay at its original value. Do not be confused by the fact that there is nothing in the box. Your password is not displayed at all.
Alternatively you can leave the password as I set it and either save the email with it written on or write it down somewhere else.