Posts that you make on a forum can be modified or deleted in any topics that are not locked. This can be useful anytime you need to correct a mistake that you have discovered after a post has been made, or you can opt to remove your post entirely from the topic if needed.
Modifying a post: If you need to go back and correct a mistake, or add more info to your post, find your post within the thread you posted into and then click the Edit button found in the top-right hand corner of your post. You will then be brought to the traditional posting page where you can make the corrections or additions. Once the corrections or additions have been made, simply click the Save Changes button and your changes will be made to the post. Once you've returned to the topic, you'll notice a "Last Edit" timestamp appear in the bottom right-hand corner of your post, letting other members know that changes were made and that they may need to re-read the post to view your changes.
Deleting a post: If you ever need to go back and delete a post that you made on the forum, this can be done by visiting the topic that houses the post that you made. Once you've found your post, click the icon located in the top-right hand corner of your own post, and then choose the Delete Post option from the drop down menu. Once this is done, your post will be removed from the thread.
If you do not find either the Edit button, or the Delete Post button depending on the action you're wanting to do, then it is likely that the topic your post is located in has been locked. When a topic is locked, no posts can be added, removed, or modified from a thread. If this is the case, then you will want to seek a forum staff member to have them make the modifications you were wanting to make, or delete the post from the forum.