Just click the advanced tab bottom left, and chose to accept the risks,
as long as our sure ubuntu.com is the site your visiting and not some site pertaining to be ubuntu.com from a link you clicked.
Also make sure you typed
https://ubuntu.com/download
that might make the error go away, sometimes old links were created using http:// , you will be warned if you try and visit http links because the connection is not secure.
Keep your browsers up to date too, I started a computer after it sitting
for Five years, could not access most sites because browser certificates were out of date. Didn't help the bios was an another half a decade behind in date and time.