should be in the gameprefs.pref file in the game directory. mine is at C:\Users\Public\Games\Cryptic Studios\Star Trek Online\Live\Localdata and can be opened/edited with notepad.
Not there for me. Using the Steam version and there is a file of that namein a localdate folder under the Steam path, but the login name listed in the file isn't the one the game keeps suggesting. Need to find what'S overriding it and fills in the wrong name every time.
I and a lot of other players no longer have this problem, so it might not be identical to yours.
Basically, if we had a Cryptic ID, then switched to PWE... it would remember the old Cryptic UserID instead of the new PWE version. Even though we edited the 'stored' useID, it would revert back to the incorrect version because the game client retrieved a different userID from the game account server.
This finally went away, late last year.
Anyway, the userID is stored locally in two places:
<INSTALLDIR>\StarTrekOnline\Live\Localdata\Gamep re fs.Pref
In the file, look for:
If the game client STILL retrieves the UserID from the account server, editing the above won't help.