Whenever you post or share a link in Facebook, it is being cache on their server and updates will not display right away when there is a changes on your post.
To Clear the cache, Facebook has a tool called "Debugger" (formerly known as URL Linter). You may use this to clear the cache in Facebook.
- Go to "https://developers.facebook.com/tools/debug/og/object/"
- Login with your Facebook account
- Copy and Paste the URL you wish to clear in the "Input URL" field
- Click "Fetch new scrape information"
Once completed, it will now the display the changes or update on your Facebook post.