How to update macOS when the system thinks the update is already installed

I've been using a 2010 iMac with OS 10.9 for a few years. Now I need to update the OS to 10.13 (the latest one this system supports).

Here's the catch: months ago I attempted to install 10.13 on an SD card plugged into the system. The install failed, possibly because it ran out of space on the card. But now when I go to the App Store and try to download 10.13, it tells me the version is already installed. It's not! It still boots 10.9 from the internal drive.

Removing the SD card and rebooting had no effect. I tried reinstalling the OS in Recovery mode and it said it was unable. So now I'm in the process reverting to 10.9 from Time Machine.

I presume there is some sort of config file lurking somewhere, left over from my failed install to the SD card, that tells the system I have 10.13 installed.

I've searched for instructions on making a bootable High Sierra installer on a USB stick but what I've found seems obsolete and doesn't work.

I seem to be stuck between a rock & a hard place. Any ideas? (I read recently that installing Catalina on an external drive would corrupt your main drive; am I looking at something similar here?)

Spent a fruitless hour on the phone with Apple support—nothing I tried had any effect. Finally solved it by downloading the High Sierra installer on another, newer Mac, and using its Terminal's createinstallmedia command to make a bootable USB thumb drive installer. Transferred this to the target iMac via sneakernet, and was able to run it and install High Sierra.