the latest version your Mac can upgrade to is high Sierra, get the installer from Apple here : https://itunes.apple.com/us/app/macos-high-sierra/id1246284741?ls=1&mt=12
then plug in your USB into your other mac where you downloaded high Sierra, then format the USB as macOS Extended (Journaled). Then open terminal in applications>utilities and type (assuming your installer is in the applications folder:
sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
Replace my volume with your volume name