I have multiple Macs, and need to upgrade each of them to Mac OS High Sierra. I really don't want to download the installation files, which are quite large, onto each computer. I did upgrade one, and wasn't able to find the upgrade or installation files once I was done to copy them to a different computer. Is there some way to reuse the installation files I download to upgrade all of the computers?
When you upgrade a mac you download the installation files from the App Store.
Once you perform the installation your MAC, deletes those installation files and they're gone so there's not much you can do in this case. On the other hand if you just download the upgrade installation files but don't run the installation then you have an option to create a bootable U.S.B. drive from which you can upgrade all of your Macs.
This is a simple process just takes a few minutes. You need a U.S.B. drive that's at least eight gigabytes in size and those are really cheap now. What you need to do is search online for create a bootable U.S.B. installation media for MAC O.S. X and you'll find several solutions you can do it manually.
There are tools that help you do it where you can get a professional to help, but in any case it's really simple and once you have that you can install the latest version of the operating system from that U.S.B. drive. That'll save you having to download the installation files on each computer.