If your message has been processed and is queued to send:
Once a message/card has been processed and is queued for sending, it cannot be changed. If you need to make an edit, the only option is to copy/paste the text from your message into another program (like Word, email, Notes, etc.), delete the message from the queue (you can find instructions on how to do that here). Then compose a new message/card, paste your old text (that you have in Word, email, Note, etc.) back in and add/delete/edit whatever you need from there.
If your message has not been processed and queued to send yet:
If you have not clicked the "Process message" button (outlined below in the red box), you can click the "Edit message" button at the bottom of the page to go back and edit your message or card. Again, this can only be done if the message has not been processed (so if you just wrote the message or if it was saved as a draft).