HG: cannot reset the username in my http authentication

I mistyped my username once for a http based BitBucket Mercurial connection.

Now SourceTree does not allow me to enter the username any more.

This is while pushing changes to a Mercurial Repository that requires login.

Restarting SourceTree does not work
I'm not allowed to run credential storage sniffer tools.

I'm behind a CNTLM proxy that works (I can push from the command-line hg fine, which also goes through the same CNTLM proxy).

Please make SourceTree enable both username and password when re-entering credentials.