Facebook automatically chooses an image from your website and a description based on the content of your site. But you can also tell them exactly what you want displayed.
To edit your Social Sharing information please follow theses steps:
1. Click the Pages link on the top menu or the Plus + button on the page tabs bar to open the Pages & Navigation window.
2. Click on the settings icon for the page you want to update
3. Use the fields highlighted below to edit the information displayed on Facebook.
When uploading your "Page Image" use images that are at least 1200 x 630 pixels for the best display on high resolution devices. At the minimum, you should use images that are 600 x 315 pixels to display link page posts with larger images. Images can be up to 8MB in size.
4. After adding your Page Title, URL and Image you can also click on the SEO tab to edit your Page Description:
5. After saving your changes, be sure to publish your site.
6. You can test your new data with Facebook's Sharing Debugger.
7. If Facebook does not automatically update with your new info, click the "Scrape Again" button to force the update