Facebook has recently removed users ability to edit link previews, so this feature is no longer available.
This article covers how you can edit what's included in a link preview in your post when posting to Facebook Pages, such as the image, copy and title. However, this is currently restricted to domains verified by Facebook.
Please ensure you have verified your domain on Facebook Business Manager and you've assigned it to the Facebook Pages you would like to edit the link preview information for.
To edit your link preview:
1. Click the Compose button on the top right of the screen
2. Select your Facebook Profile and paste in a link in the compose box
3. Go to the Facebook tab and hover over the text to edit the link preview
Please see our article Why Is My Link Preview Metadata Not Appearing on Facebook?