If you have uploaded a new share image or changed the text for the share button, but it hasn’t updated on your Facebook page yet, you may need to refresh the app with the Facebook debugger tool. This is a simple tool that will take your offer URL and pull all of its data to the Facebook database.
The tool is located at https://developers.facebook.com/tools/debug/. Just paste in your shareable URL and click Debug. The share button should be updated instantly.