For the record: that rssing.com site seems to be a scam site re-using content from other sites (in this case: from sdb.univention.de) trying to sell their own ads. Please don’t post links to such sites. Instead links to the original content provided by Univention.
Recently Univention has moved content from the old support database site (sdb.univention.de) to this help forum. You can find all support database entries via this link. The original “SSL certificate renewal” article is this one.