To reset you password go to www.ecmwf.int and click Login.
On the Login window you will see a "Forgot Password" option, click on there
Then you , provide the username or email of your account
You will receive an email with a link , click on it
By clicking on that email you will be automatically logged into www.ecmwf.int , now, you can change your password by clicking on your name on the top right of www.ecmwf.int
Following that you will reach the page to change your password . Update the password as you see fit.