The link should work you need to open your Safari browser, Firefox will not work.
How to upgrade to macOS Sierra - Apple Support
Go to Section 4 and click on Get macOS Sierra
this will download the InstallOS.dmg which you then open to
InstallOS.pkg which in turn you open to an installation window,
this does not install macOS Sierra but creates the Install macOS Sierra.app
in your Applications folder, use that to install Sierra.
If your mac can upgrade to Sierra it can upgrade to macOS High Sierra.
Again use your Safari browser and click on the link below.
How to upgrade to macOS High Sierra - Apple Support
Go to Section 4 and click on Get macOS High Sierra
this will connect you to the App Store High Sierra download page
it should say Get or Download.
Before upgrading make a full back up of your Mac.
Check that third party apps you use have updates available for the new OS.
https://roaringapps.com
Check that your printers/ scanners etc have updated drivers and software available for the new OS.
Connect your mac to your router via cable rather than rely on WiFi.
Do not use your mac while it is downloading
and do not use any other devices that share the same internet connection.
If the App Store says ‘Open’ then it is detecting a previous download of the install app
go to the Applications folder locate Install macOS High Sierra.app.
Move it to the Trash, empty the Trash.
Restart, click on the link again, the App Store should say ‘Get' or 'Download'.