Mở Tin nhắn iPhone. Nhấp vào Edit (Chỉnh sửa). Chọn đoạn hội thoại muốn xóa. Nhấp vào Delete.

I'm interested in that, but I only have a few mins. Can you give me at most the first 500 characters of a detailed explanation in English about that?
Do this by tapping the white speech bubble on a green background icon. It should be on one of your Home screens. This is in the top left corner of the Messages screen. If you have a conversation open, tap < in the top left corner of the screen to return to the Messages menu.  This is in the bottom right corner of your screen. Your selected messages should now be gone.