Hi Bek. :D
Are you asking how to edit a message for which you have not yet clicked the SUBMIT button? If so, you just click the PREVIEW button, and if your text isn't satisfactory, scroll back up to where you type your message and correct it. Keep previewing it until it's satisfactory.
If you've already submitted a message, then you'd click the EDIT button (in the upper right corner of your posted message), and do the same thing.
Does that answer your question?