Use Modify|Relocate to change the repository for the selected directory (and subdirectories) of your local working copy. Typically, this command is used when the URL/structure of an SVN server has changed.
Relocate Directory shows the directory, relative to the project's root directory, which will be relocated. From URL displays the repository root URL of the selected directory, if this information is available locally. Otherwise it displays the complete repository URL of the directory. With To URL you can now specify the replacement string for From URL: Relocate will then search within the selected directory and subdirectories for URLs starting with From URL and replace the corresponding part by To URL.