If you want to upgrade to High Sierra:
App Store/Featured tab.
Click on the High Sierra banner - or, "Apps Made by Apple", then click on macOS High Sierra.
Be certain to have a good backup before you begin the upgrade, just on the possibility that something happens during, or there is some problem with upgrading your drive to APFS, or you just don't like the High Sierra upgrade.
The challenge is if you upgrade your drive to APFS, and then want to back out of that. The drive will have to be erased if you want to revert, so a backup is essential for THIS upgrade, even if you don't ordinarily think about backups.