No, you can't do it in the UI. Like William mentioned, it's stored in a set of config files by the Subversion client. On Windows (Vista, 7, Server 2008) they are at %appdata%\roaming\subversion\auth. On Linux they are at ~/.subversion/auth). Of course, the passwords are all encoded so you can't change them directly, but on examining the file contents you should be able to figure which files goes with which Subversion server, delete just that file, and the Subversion client should ask for your credentials the next time you attempt to access it.
It's probably safer to delete and recreate the repository definition within the SVN Repository View GUI. Just make a note of the associated repository URL so that you can properly re-enter it. Eclipse may ask a few questions if you have projects attached to that URL, but it all works out in the end.
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