UITableViewCell edit cell content on click

I have a question about UITAbleViewCell's.

I have implemented UITableViewDelegate method

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
    UITableViewCell *cell = [self tableView:tableView cellForRowAtIndexPath:indexPath];

    cell.textLabel.text = @"TEST!";
}

After I click on desired cell, nothing happens...

Why it doesn't work as I expected? Also, what should I do to make it work?