If you are searching for OSX how to install High Sierra and you can’t find it in the apple store, it is because it is not there. It has been removed.
You’ll have to visit this link: http://updates-http.cdn-apple.com/2019/cert/061-39476-20191023-48f365f4-0015-4c41-9f44-39d3d2aca067/InstallOS.dmg which is referenced from Apple support at: https://support.apple.com/en-au/HT208202 in order to download it and then install it. Annoying yes but no choice.
It takes about 10 mins, depending on your internet connection to download and then install.