Leav, you should see an "edit" button at the bottom of the post you wish to edit. Once you click that, you can edit the post, delete the post, and give a reason for editing or deleting the post, (if you want to). I believe there is a time limit for editing posts, though I'm not sure how long it is.
For example:
The first image is what your post should look like in a thread. You can see the "edit" button next to the "reply" button. Once you click "edit" you should see the second picture.
Here, you can see the "delete this message" window. The "edit post" window is where you edit the post itself -- it looks just like the window in which the post was originally typed.
Hope this helps!