Note that the key expires in 1 year. You will receive an email to the registered email address 1 month before the expiration date with the renewal instructions. To check the expiry date of your current key log into www.ecmwf.int, then click here.
Copy the information in this page and paste it in the file $HOME/.ecmwfapirc (Unix/Linux) or %USERPROFILE%\.ecmwfapirc (Windows: usually in C:\Users\<USERNAME>\.ecmwfapirc ; see how to create a file with a leading dot)
contents of $HOME/.ecmwfapirc (Unix/Linux) or %USERPROFILE%\.ecmwfapirc (Windows)