If you click on the email in the drafts folder, right at the top of the body of the message, there is a purple button called Keep Composing Message, if you click on it, it should allow you to edit the message.
I will assume you are using BTMail and not BTYahoo.
When you save the email as a draft it obviously goes into the draft folder. If you retrieve the email from the draft folder by clicking on it there should be a button "keep composing message". If you click on that it will open the "draft" email and allow you to edit it.
Many thanks gg30340 for your reply which has solved my draft mail problem - Thanks for your time and trouble!
Many thanks for the info which has sorted my Draft problem out! Thanks for your time for getting me sorted! Regards, Paul