The only way to "change" your Yahoo! Mail email address is to register a new one. You can cancel your current account and register for a new Yahoo! Mail account. Or, if you prefer, you can keep it and simply register for a new account. Yahoo! Mail now also offer email addresses with the suffixes of ymail.com and rocketmail.com, in addition to yahoo.com.
Sign in to your current Yahoo! Mail account. To cancel this account and register for a new Yahoo! Mail account, pull down the "Options" menu located at the top far right of your email screen. A new screen will pop up. Click on "Accounts" from the menu on the left.
Log out of Yahoo! and close your Internet browser window completely. Reopen your browser and log in to yahoo.com. Click on the "Sign Up" button. Fill in the fields for your name, gender, birthday, country and postal code.
Choose a username; this will also be the prefix in your email address. Click the "Check" button to make sure it has not yet been taken by another user. Create and confirm a password for your new Yahoo! Mail account. Select two security questions and respective answers. Enter the text code and click on "Create Account." Yahoo! Mail will send you a verification code to your new email address to confirm the newly-created account. Copy and paste the code into the indicated field on the included confirmation page. After you confirm your new account, you can begin using your new email account immediately.
Prior to canceling email accounts, make sure everyone whom you want to continue corresponding with has your new email address. Also, be sure to forward any important messages to a different email address. Download all files and attachments you want to keep and save them to your computer or a portable drive. These will make your transition over to your new account a smooth one.
If you change your mind and decide you want the same email address back, you will have to re-register for it again. Any folders and files you may have had previously will need to be recreated. Note, however, that once you have canceled an account, the email address you just canceled automatically becomes subject to availability. If someone beats you to re-registering the email address, Yahoo! will notify you that it has been taken.