How do I get round this problem
Quit the installer app. The instructions state as such (emphasis added):
Download the installation files, but make sure you stop before the actual installation starts. When the installation starts the files will be disposed of, so you need to step in beforehand!
Those instructions are accurate and will work. I just tested them. Do endeavor to pay attention to detail — that's critical when using instructions found on the web, which are often wrong and tend to get people into trouble. That MacWorld article is a rare exception.
This Apple Support article also states the need to quit the installer app before launching it: How to download and install macOS - Apple Support.
After downloading to your Applications folder, the installer opens automatically. Follow the onscreen installation instructions. Or quit without installing to leave the installer in your Applications folder for later use.
That's what you need to do.
One more thing. You wrote that your Mac is slow. Upgrading macOS is rarely the cause, and downgrading macOS is rarely the solution. Go ahead and do that if you wish, but you will almost certainly be better off attempting to determine the reason for its poor performance first.
