WhatsApp allows you to mark chats as unread. However, this does not change the status of a message. If you open a discussion, the sender will still be able to see that you have read the message. This option only allows you to indicate that a discussion is important if you ever want to come back to it later.
Steps
Method 1 of 2: Use iOS

Step 1. Open WhatsApp
You must have the most recent version installed on your Apple device. If not, update the app using the App Store.

Step 2. Tap Discussions

Step 3. Slide your finger from left to right on a chat

Step 4. Select Mark Unread
A blue dot will appear, marking this discussion as unread.
Method 2 of 2: Use Android

Step 1. Open WhatsApp
You must use the most recent version. If not, update the app using the Google Play Store.

Step 2. Choose Discussions

Step 3. Tap and hold a chat

Step 4. Select Mark Unread
A green dot will appear on the side, marking this discussion as unread.