You should upgrade to at least OS X 10.11 El Capitan. Then it can handle doing modern iOS/iPadOS upgrades.
Simply download this disk image file: Download OS X El Capitan
Then open the disk image, run the package it contains, and that package will create the Installer app and put it into your Applications folder to upgrade your system to 10.11.
If you want to, you can further upgrade to macOS 10.13 High Sierra, but that shouldn't be necessary to upgrade your update. Although it's not necessary though, it is a good idea.
This has the link for High Sierra on the App Store but note, you must upgrade to El Capitan first:
https://support.apple.com/en-us/HT208969