It sounds cruel to say this, but essentially? They flail.
Unless they’ve been raised in a cave, they’ll be affected on some level by what they’ve seen work around them, be that the local militia or television. There’s going to be a fair amount of hair grabbing, if their opponent’s hair is long. They’ll push at their opponent, though if their opponent is larger it’s not going to do them much good. An untrained person, for the most part, is at the mercy of someone larger and heavier. They just don’t know what to do with the weight and it’ll crush them. They’ll flail against it, but again, they won’t know how to do harm so the lasting damage they can put out is minimal.
They’ll smack at them, either with slaps or some variation of closed fists, but they’ll be using their upper body, for the most part only their arms and shoulders. Which, again, won’t do them much good. They’ll go down, usually on the first or second solid hit.
Human instincts are shit on their own, I’m sorry. A human gets caught between their fight or flight response and usually against a tougher opponent, their first instinct is going to be “huddle up in a small ball and wait for it to be over”. This is true of both men and women, by the way. There is no magical genome in the human body, buried in our ancestral memories that can tell us how to save ourselves in a bad situation.
I don’t know if that helps.