How do i delete MacOS High Sierra

Select "About This Mac" from the Apple menu and read what it says. macOS or OS X is the operating system, not what Mac you have. Do you have a MacBook or an iMac or a MacBook Pro etc. and what year?

Can you add some detail? You installed High Sierra and some of your apps won't run or, your Mac won't start etc?

Speaking of, what Mac do you have?

I see. What year and what version of OS X are you running? Did you get an error message?

You might want to read this…

Writing an effective Apple Support Communities question