This article applies to your 123-reg account.
The username of your 123-reg account cannot be modified once created.
It is possible to setup a new 123-reg account on the system under a preferred username.
You may then transfer your domain name(s) via the internal transfer function through the control panel from the old username account to the new one.
Please note: Only domain names can be transferred between accounts. Please do not transfer domain names if you have other products associated with them.