Subclipse does not collect or store username and password credentials when defining a repository. This is because the JavaHL and SVNKit client adapters are intelligent enough to prompt you for this information when they need to — including when your password has changed.
You can also allow the adapter to cache this information and a common question is how do you delete this cached information so that you can be prompted again? We have an open request to have an API added to JavaHL so that we could provide a UI to do this. Currently, you have to manually delete the cache. The location of the cache varies based on the client adapter used.
JavaHL caches the information in the same location as the command line client — in the Subversion runtime configuration area. On Windows this is located in %APPDATA%\Subversion\auth. On Linux and OSX it is located in ~/.subversion/auth. Just find and delete the file with the cached information.
SVNKit caches information in the Eclipse keyring. By default this is a file named .keyring that is stored in the root of the Eclipse configuration folder. Both of these values can be overriden with command line options. To clear the cache, you have to delete the file. Eclipse will create a new empty keyring when you restart.
Note: If using tortoise SVN, can reset the same by going to settings > Saved Data and clear Authentication Data.