Update from 10.13.6 to 10.14.6 High Sierra - how can I do this?

Everything I've googled doesn't work or its not there or on App Store, any ideas, I want to put logic on my system but theres only the new logic you can buy it seems so I need my OS to be 10.14.6 from 10.13.6 or have the ability to buy and install an older logic version that is compatible