You can only edit your own posts. You can do so by selecting the relevant post and clicking 'Edit Post' under the post. Clicking this link takes you to a similar form to that for the original post. Having made your edit, a reason for the edit is required.
You can change the title, content, group and MetaData for a post. However, deleting posts is intentionally disallowed; if you find you have made a mistake you should edit your post clearly to show this.