To edit a pin you need to open the Pinboard tab and right-click on the pin you wish to edit (or click on the tiny blue arrow in the top-right corner of the pin).
From the drop-down menu that appears choose "Edit". Once you’re finished with the editing bit, hit "Save" and you’re good to go!
To remove or delete a pin from the Pinboard you need to unpin it. To do this, open the Pinboard tab and right-click on the pin you wish to delete.
From the drop-down menu that appears choose "Unpin". Once you’ve done this, the pin disappears from the Pinboard.
Alternatively you can do the same action in the conversation, in the message flow, on the initial message that was pinned (it can be found by using the Show in conversation option on the pin).
Note that every member of the conversation can Edit or Unpin the pinned messages on the Pinboard.