Before proceeding, verify that your Mac supports macOS High Sierra (version 10.13.6). Generally, any Mac that supported macOS Sierra can run High Sierra.
If you already have an earlier version of High Sierra (like 10.13.0 or 10.13.5), you can update directly to 10.13.6 using standalone update packages from Apple Support : download+macos+high+sierra+10136+dmg+mac+install