Before this latest update, your only recourse for correcting a typo or an otherwise erroneous comment on Facebook was to remove the comment and start again. (Or, for longer comments you didn't feel like retyping, you could always copy them before deleting to paste and then fix.) Now, Facebook is giving users the ability to edit their comments.
When this new feature hits your page, instead of seeing the X button to remove a comment, you'll see a pencil icon, which gives you the ability to edit or delete a comment. To preserve transparency, any comment you have edited will have a gray Edited link below it, which will show the revisions you made.