If a page has been shared on Facebook, Facebook will cache the image and meta data for the page being shared. If some of the information is incorrect or you update some of the information on the website being shared, even if you share the information on Facebook again, the information will not be updated on the new Facebook post. To get the information to update you need to go to the following link
https://developers.facebook.com/tools/debug/sharing/
Enter the URL that you want to share and click Debug.
Once the page loads then you would need to click the Scrape Again link and any subsequent times you share the link should be updated