In our System the Customer Username for Logging in is an Email Id. You will receive all mails from the System on this Email Id. Hence, it is very important to keep this as a valid Email Id. You can change this Username from the Control Panel by following the below mentioned steps -
1. Login to your Customer Control Panel
2. After Logging in go to Settings -> Primary Profile.
3. There you can change your Email Address and then Submit. You would have to use this as your Username the next time you Login.