I am currently running my iMac late 2015 on original Sierra OS. Can I update to Mojave or must I go to

Software Update and the App Store will not allow you to upgrade to any version of macOS except the most recent version. Today that is Catalina. You may be able to work around this by trying the following. Choose one or the other. Because you are running an earlier version of macOS, I think you will be able to download Mojave from one or both of these links.

macOS Mojave

https://itunes.apple.com/app/macos-mojave/id1398502828?mt=12

https://apps.apple.com/app/id1398502828?mt=12

This is what I get (and, FWIW, when I picked it up at the store, it did have Mojave loaded; I then talked the Genius into flashing it for me and installing Sierra from their server - which he did after arguing for a while that it couldn't be done). I probably could have done it using my clone, but wanted to walk out of there with the OS I wanted.

Anyway, here is a screenshot of what I get:

Wait a minute - just checked my Applications folder and both HS and Mojave installers are already sitting there, so that may be why it won't let me download it.

So. never mind your charges, try that on some other xxxxxx, LOL. Good try though!

I just got a refurb 2017 iMac which is still running 10.12.6. Thought I'd try your links since I will want to do some updating - at least on my Mac, neither of them goes further than "this item is not available in the US store".

You may want to check your Applications folder for any existing installers. Put them in the trash until you download what you need. I won't charge for this one. It's a freebie.

If you don't try, then you don't succeed, right?!

Both links work for me but then, I upgraded to Mojave in the past. Maybe that has something to do with being able to reach the Mojave download in the App Store.

Should work, babowa. I only charge a small fee for each. I'll add it to your tab.