Instagram does not offer a built-in feature to undo or revert the change in message status from "unread" to "seen" once you've opened a direct message (DM). Once you've viewed a message, it will be marked as "seen," and this action cannot be undone through the Instagram app.
If you've accidentally opened a message and want to maintain the appearance of it being unread, you may consider these options:
Turn Off Read Receipts: You can disable read receipts in your Instagram settings to prevent others from knowing when you've read their messages. However, this will also disable your ability to see when others have read your messages.
Use Airplane Mode (Limited Success): Some users have reported that if you open a message while in airplane mode (with Wi-Fi and cellular data turned off), Instagram may not mark it as "seen." However, this is not a guaranteed method, and Instagram may have updated its functionality since then.
Contact Instagram Support: If you believe there was a technical issue or a bug causing the message status to change unexpectedly, you can try reaching out to Instagram's support team through the app for assistance. They may be able to help, but there are no guarantees.
Please note that Instagram's features and functionalities can change over time with updates, so it's a good idea to check the latest Instagram Help Center or community forums for any new features or methods that may have been introduced