You can edit any post, story, update or survey response that you have created.
If you are an Organisation Admin, you can edit any story within your organisation
Posts, Updates and Survey Responses are all Stories.
Steps
Go to the Story page of the story you want to edit
Press the More Options button on the right
Select Edit Update
You arrive on the Edit Story page
Make the changes you need to make
Scroll to the bottom and click Publish