How can I delete just 1 or 2 message
Click on the person's name from the Message screen. This will take you to a record of all the messages you have exchanged with that number. 
Click on the edit button at the top right of the screen.
Choose the message or messages you would like to delete. If you change your mind and decide not to delete any, simply go to the "Done" button at the top right of the page. 
Once you've finished selecting all of the messages you would like to delete, just tap the "Delete" button at the bottom of the page!