You are only able to delete a comment you made on BTWB. This can ONLY be done from the website. NOT the App. You are unable to edit a comment.
To delete a comment, go to the members workout you commented on, and select "Comment" on the bottom of the WOD.
From here, scroll down to the bottom of the page. Next to the comment, you will then need to select "Delete". This will remove the comment.