Changing og:description on WP site

Hi,
I'm wondering if you're able to help me - a couple of people have asked me how they can change what facebook automatically displays as the exerpt when linking their website.
I've checked on the developer sharing debugger on Facebook (https://developers.facebook.com/tools/debug/sharing/?q=feltfusion.co.uk) but other than seeing that it's the og:description meta tag that needs changing, I'm not sure how to proceed. I'm guessing I just need to change the information on one of the pages, but I'm not sure which and I don't really want to go poking around without being sure. Alternatively, is there a plugin that can assist?
I'd really appreciate any guidance on this!
Thank you, Stacey