Yes, you can directly update the parent theme.
It is not recommended because the changes you will make will be gone with next update.
If you need a minor change, you can update the CSS code in the parent theme using custom CSS. Once it works, copy the code and save in notepad file. Now, whenever the theme updates, you can put the code back. It is easier than maintaining the child theme.
Generally, the theme structure is not changed with the update. But even if it does, the worst could be the custom design won't render with the previous CSS code.