If you are not a trained first aider, then I strongly suggest you did a course as soon as possible. As David Liepmann said, the recovery position is generally safe. However, if you are not a trained first aider, you might miss either something or do the wrong thing or exacerbate things that will lead to the victim dying.
I am not a lawyer, nor do I play one on TV. That said, if you have rendered someone unconscious in a fight, it is your responsibility that they do not die. That is true even if they attacked you first. It means that you must contact the emergency services (112 should work in most countries) and ensure that the victim stays alive. For that, I strongly suggest being trained in first aid. Otherwise, you risk being arrested and charged with murder. Whatever, never talk to the police but talk to your lawyer as soon as possible.
If you wanted an authoritative answer, I would talk to some paramedics and lawyers.