How do I move to older version of Mac OS?

This is what you do: Create a bootable installer for macOS - Apple Support. Sierra is not listed on the site.

It is listed here: How to download and install macOS - Apple Support

But, that's a lot of manipulation to get an old app to work. Is there no later version of the App?

You don't have details about your Mac, but if it's so old you want to downgraded to very old OS, you might want to consider a new, or newer, model.

You also need to have an older Mac that can run the OS version you want. Macs are not backwards compatible. The easiest way to get there is to have access to one running Sierra. Create a partition to run Sierra on an external drive. Use CarbonCopyCloner to clone the Sierra to the external and then use that to clone it to your other Mac after creating a partition for it. Test it to make sure it boots.

I have a 2017 iMac which came with Sierra; I created a bootable clone on an external as soon as I got it home. I have since updated it, but I can boot into it when I want to run the older software.