This app resembles a white bolt of lightning on a blue background. If you aren't signed into Messenger, type in your phone number, tap Continue, and enter your password. It's a house-shaped tab in the bottom-left corner of the screen. Older messages will be further down the page, so you may have to scroll down. The further up the page you scroll, the older the messages are. This will open it, allowing you to read it.

TL;DR in Vietnamese:
Mở ứng dụng Facebook Messenger. Nhấn thẻ Home. Nhấn vào cuộc trò chuyện. Kéo tin nhắn xuống. Nhấp vào tin nhắn mà bạn muốn đọc.