Unfortunately, at the moment, the sent letters cannot be edited or deleted.
Our original idea is to make it closer to real life experience - once you posted a letter, you can't take it back; and also, to avoid confusion from the receivers - they will receive notifications once you sent the letter.
However, after listening to more feedbacks, we understand that it may cause potential problems to users - as there are accidents. So, we are going to enhance this feature in the future. Please stay tuned for our news.
Meanwhile, is it possible to send another letter to your pal to explain?