Keep in mind that once you share an ffm.to URL on socials like Facebook and Twitter, they will cache the original images and meta-data, so even if you change them, it could take some time for Facebook or Twitter to register.
Facebook offers a solution to re-scrape new URLs so you can re-share with the new meta-data, however.
For Facebook, you can go here: https://developers.facebook.com/tools/debug/ and then press "Debug" -> "Scrape Again"
For Twitter, you can learn more about their process here: https://developer.twitter.com/en/docs/tweets/optimize-with-cards/guides/troubleshooting-cards and validate your Twitter Cards here: https://cards-dev.twitter.com/validator
What else can I do?
In addition to re-scraping your link, you can also take one of the below actions and Facebook / Twitter will scan your link again when you share
The below actions will actually cause Facebook and Twitter to treat your link as new URLs.