Can I modify the canonical tag of an article, or any other Guide page?
No, the canonical tag of an article can not be changed. Every Guide page is using a canonical URL tag that is set up automatically once a new page has been added. As this tag is configured on the Help Center's back end, the canonical URL can't be changed manually. Due to this reason, a new canonical tag can't be added against an article, either.
More details about SEO can also be found here: About search engine optimization (SEO) in Help Center.