It's not possible to change a message already sent. The reason for this is that it may have been read and by amending it, users who have already read the message may not see the changes.
You can however delete a message and then send a new one. This way the user will receive a notification that a new message has been received and the new message will be highlighted in their inbox.
To delete an existing message, go to that message on your phone and swipe left. Three options appear - 'Delete', 'Mark as Read' and 'Mark as Unread'. Selecting 'Delete' will remove the message.
Now you're free to create a new message secure in the knowledge that the users will see it as new.