JSFL does not have a method to rename a folder, which means you have to create a new folder with the new name, copy all the contents of the old folder into the new one, and then delete the old folder. It's a bit of a pain.

Here is the JSFL code to do this until Adobe sees fit to add a rename folder method. You can also use this to move a folder from one place to another.

function renameFolder(sourcePath, targetPath)
{
    FLfile.createFolder(targetPath);
    copyFolders(sourcePath, targetPath, FLfile.listFolder(sourcePath, "directories"));
    copyFiles(sourcePath, targetPath, FLfile.listFolder(sourcePath, "files"));
    FLfile.remove(sourcePath);
}
function copyFolders(sourcePath, targetPath, folderList)
{
    var i = folderList.length;
    while (i--)
    {
        FLfile.createFolder(targetPath + "/" + folderList[i]);
        copyFiles(sourcePath + "/" + folderList[i], targetPath + "/" + folderList[i], FLfile.listFolder(sourcePath + "/" + folderList[i], "files"));
        copyFolders(sourcePath + "/" + folderList[i], targetPath + "/" + folderList[i], FLfile.listFolder(sourcePath + "/" + folderList[i], "directories"));
    }
}
function copyFiles(sourcePath, targetPath, fileList)
{
    var i = fileList.length;
    while (i--)
    {
        FLfile.copy(sourcePath + "/" + fileList[i], targetPath + "/" + fileList[i]);
    }
}