You can delete any post that is NOT the first post of its thread.
First click on "Edit Post" which appears near the lower right corner when you are looking at one of your own posts.
Then click on "Delete" which will appear near the lower right corner of new editing window which you will then see.
A new small tab will appear below the editing window. You need to do two things:
1) Click in the little circle to the left of "Delete Message" on the left side of this new tab.
2) Click on "Delete Post" on the right side of this new tab.