Can anyone tell me where on the Craft dashboard I can correct this and upload new keywords?
So this isn't a simple answer, because we don't know how the SEO has been implemented on your particular site. It could have been done manually, or they could have used my SEOmatic plugin or it could have been another plugin. You also don't mention what version of Craft CMS you're using.
In any event, what that SEO tool is telling you is that the
<meta name="keywords"> tag is largely ignored by Google. I say largely, because if you stuff it with all sorts of keywords that look "spamy" it can be a negative ranking signal for your site.
Other search engines such as Bing, Yandex, etc. do look at the meta keywords tag, though, and as long as you're not "keyword stuffing", there's no harm in leaving that tag in there.