An important link you will want to bookmark is: https://developers.facebook.com/tools/debug/
This tool will show you what content FB will put in the share card based on it’s cached content (if the link has been shared previously), or a new scrape of the page data.
Importantly
If you find that Facebook is outputting out-of-date or otherwise unexpected data for a particular URL you use the above tool to clear FB’s cache. Paste the page URL in just as if you were debugging it normally, but append the parameter fbrefresh=ANYTHING (the value of the param can be anything, not important) to the end – this forces FB to re-crawl and re-cache.
Further information