What is the correct action to take to change the evil of seeing a young man and woman sitting intimately close to each other in a dark place, and how can change by hand be applied in this situation?

The situation described between the young man and the young woman is one of suspicion and must be denounced. The one responsible for discipline should not immediately discipline them for fear that the young woman might be one of his maharim (unmarriageable kin). Instead, he should denounce them both and say: "If she is your mahram, then protect her from suspicious situations. And if she is a stranger, then beware of seclusion (khalwa) that could lead you to disobey Allah, the Almighty." A believer must avoid doubtful situations, and enjoining good and forbidding evil is a communal obligation (fard kifaya). This is done through the known steps, including separating them, while observing the Sharia-compliant guidelines for enjoining good and forbidding evil.