When you make changes to your site, those changes do not instantly update across the internet. Given time, different programs like Google or Bing or Facebook will update to those changes depending on when they crawl the internet for updated data. 

There are tools such as the Facebook sharing debugger that you can use to force updates.

Simply insert the URL of the page you're working on and click debug. Afterwards you'll be taken to a new screen that'll give you a breakdown of your page as it relates to your Facebook shares. If the Link preview has not updated, hit scrape again. You may need to do this a few times. 

You may also want to clear your own browsing cache or try using a different browser if you're not seeing the update. 

Note that Twitter has a similar tool called the Card Validator.