It's possible to request a Community username / login name change.
I'd suggest searching the community, select Members from the drop-down list and see if your preferred login name is available. The login name must be between 5 and 25 characters using multicase letters, numbers, dash and underscore.
Then update your Community Profile Personal Information (Click here) scroll down and add the proposed login name to Private notes and save changes. The TalkTalk Community Team OCE will then be able to pick that up and, if suitable, change it for you.
We can change your Community Username for you, however as Gondola has advised it would be worth checking first to ensure that your preferred username isn't already in use. If you let us know what you'd like this changed to then we can update this for you.