Deleting entire text stream from one person vs one at a time as previously you could.

To delete an entire conversation, on the List view swipe from right to left and tap the Trash icon, then confirm the delete in the resulting dialog. Note that this has not changed from previous versions.

I told you how to do it. Read what I posted. It does exactly what you want to do.

Delete a single message

Within a conversation, touch and hold the message bubble to open an options menu.Tap More . Tap the Trash button , then tap Delete Message..

Delete and recover messages on your iPhone or iPad