how to delete all imassage within a group chat without leaving group

This is the way it has worked for me in the past: Open up the message thread. Press and hold on one of the messages until you see a bubble slide up from the bottom of the screen that says "Copy" and "More". Tap "More". Select all of the messages in the thread by tapping on the button to the left of each message. Then, tap the trashcan icon in the lower left of the screen. The messages will be deleted but the thread will remain.

I don't have any group chats that I want to delete the messages on to experimentnt with .

odyssey2003 wrote:

I understand and know how to do it, if you select all the messages to trash it and you won't exist the group and that how it work in ios 10, but not in ios11.

But you still not clear what I am asking for, thanks

I'm perfectly clear about what you're asking. I just don't have the answer. It's possible that Apple has simply removed that little quirk.