Hi, craigfromsioux falls....assuming your old Mac book is working OK, go to the large Apple icon, top left of your screen, then click on "About this Mac", you should look for a Mac identifier, eg " MacBook XX,y, with the XX being 2 numbers/digits, and the y being another one, or look on the outside of your Mac book's case for an official model number, which looks like this "Axxxx", where the x's are 4 digits/numbers, write down/.do a screenshot, remember both the Mac Book Indentifier in step 1 and the Mac book model in step 2 and go to everymac.com to find your model of Mac Book and what the Maximum (latest)version of the Mac OS it will support....Each version of the Mac OS has requirements. If your Mac meets or exceeds those requirements, then you can run that version of Mac OS, if not, then no, you can't, and thus you can't update... Also, bear in mind that if you try using Software Update to find updates for the Mac OS it won't work that well anymore.... best to try this way to get Mac OS upgrades/Updates (see link(------->>>>
How to download and install macOS - Apple Support (CA)
that link will have almost every version of the Mac os available for download....you should use Safari for downloading. Other browsers will probably work, but you'll have to point them to the App Store (for High Sierra or later) or the other one....
John B
