Renaming all files and directories in NSLibraryDirectory

You could use this to check if a NSUrl is a directory or file:

for (NSURL *item in directoryContent) {
    NSString *path = [item path];
    BOOL isDir;
    if ([[NSFileManager defaultManager] fileExistsAtPath:path isDirectory:&isDir]) {
        if (isDir) {
            NSLog(@"%@ is a directory", path);
        } else {
            NSLog(@"%@ is a file", path);
        }
    } else {
        NSLog(@"%@ does not exist", path);
    }
}

UPDATE:

To rename all contents of your directory you could use this:

NSArray *directoryContent = [fileManager contentsOfDirectoryAtPath:path error:nil];
BOOL isDir;
for (NSURL *item in directoryContent) {

    if ([[NSFileManager defaultManager] fileExistsAtPath:path isDirectory:&isDir]) {
        if (isDir) {
            // create a new directory with lowercase name,
            // move contents of old directory to the new one
            // then delete the old directory
        } else {
            [[NSFileManager defaultManager] moveItemAtPath:path toPath:[path lowercaseString] error:&error];
        }
    }
}