how do I download 10.15 to upgrade 10.10

Choose Applications from the Finder's Go menu, throw away the existing installer, and download a new one. If you continue to get that message, restart with the Shift key held down, and if possible, use a wired Internet connection.

Opening the link in Firefox will produce a different error message or just be ignored.

(193492)

Remember that older expensive apps such as Office 201 and older, and older versions of Adobe Creative Suite, will not work under macOS 10.15 Catalina. If you have older apps that you cannot upgrade, consider installing macOS 10.14 Mojave instead.

Mojave is here: https://apps.apple.com/us/app/macos-mojave/id1398502828?mt=12

Back up your data, check your applications for compatibility, and download Catalina through the link provided. It won't show up in the App Store unless accessed from elsewhere.

(193431)

Don't use Firefox.