You can edit and delete posts you've made to the Announcements, Crowd Pics, and Social Feed page. The process is the same for both posts and comment.
Tap the '...' menu for the post/comment in question
Tap "Delete" from the menu the comes up.
If you are the app's admin, you can also delete other people's posts and comments. You must be logged in as the app's creator to do so.
Please note that if your app is subject to feed moderation (i.e. the app creator has turned feed moderation on), you will not be able to edit your comments or posts.