Subclipse does not own the information about users and passwords (credentials), so there is no way for you to manage it from Subclipse itself. It is controlled via adapter Subclipse uses (SVNKit or JavaHL).
The adapter can be choosen in the subclipse preferences (Window - Preferences - Team - SVN -SVN Interface).
SVNKit stores credentials in Eclipse "keyring" file - default encrypted storage for credentials, you may find it for Windows at ECLIPSE_HOME/configuration/org.eclipse.core.runtime/.keyring or on Mac OS X and possible other *nix distros at ~/.eclipse_keyring
JavaHL uses Subversion credentials storage, which files could be found below SUBVERSION_CONFIG_DIR/auth directory. On Linux and OSX configuration directory is ~/.subversion, on Windows it is "C:/Documents and Settings/USER_NAME/Application Data/Subversion".
To delete cached password information you can delete the keyring file (in the SVNKit case) or the contents of the auth folder (in the JavaHL case). The next time subclipse connects to a repository, you will be asked to provide the username and password again.
In the case you delete the auth folder, instead of the files in it, you will stumble upon a Subversion bug in JavaHL, where it will not recreate the proper structure. So now it is not able to cache these values. If you have access to the SVN command line, just run a command and it will create the structure. Then you should be back to normal.