You can change or remove anything you posted yourself — posts and comments alike — on a Social Feed, Crowd Pics or Announcements page. The steps are the same either way.
Tap the three dots menu on your post or comment.
Tap Edit to change it, or Delete to remove it.
On a grid layout, the three dots are in the top right of the photo.
If the option is not there
Some apps hold posts for approval before anybody sees them. While a post of yours is waiting to be approved, you cannot edit it. Once it is approved it behaves normally.
You also need to be signed in as the same person who wrote it — see signing in to post.
Related
Blocking and reporting objectionable content — if the problem is somebody else's post