Hello;
Unfortunately it is not possible to change the user name for a Larian account. However, if you delete the account you would be able to recreate it with a new user name, and optionally the same email address.
Deleting the account will automatically disconnect any connected accounts, so you would be able to link them again to the new account.
If you log in to the
Larian account website in a browser, when editing the account there should be a link near the bottom to delete it (on a grey background, along with an option to log out of all devices).
The only data that could be lost, other than the account information itself, would be the cloud copies of cross saves, if that function was enabled. The local saves and any platform cloud saves would not be affected.