Hi there!
+1 that OS X El Capitan to macOS Catalina is the best upgrade path. OS X El Capitan includes an updated App Store that might make the macOS Catalina download more reliable. However, before installing new software, it's a good idea to back up your Mac.
As mentioned, you can visit the Apple Support article How to download and install macOS - Apple Support, and under the "Use a web browser" section, choose El Capitan 10.11. This will download an installer, placed in your Downloads folder, to install the OS X El Capitan installer application. Open it, and follow the on-screen steps to install the application. After it's installed, you can open the app "Install OS X El Capitan" from the Applications folder in Finder, or Launchpad, and begin the upgrade.
After OS X El Capitan is installed, you can upgrade to macOS Catalina. Go back to How to download and install macOS - Apple Support, but this time, look in the "Use the App Store" section to locate Catalina 10.15. Choosing it will open the Mac App Store, and from there, you can download the installer application. Once its downloaded, it should open automatically and you can follow the on-screen steps to upgrade your Mac.
Hope this helps! If you have any questions, let us know!
Jack