Apple hosts manual "Combo Update" files (like the 10.13.6 Combo Update ) on its support site. Note that these are updates and require a base version of High Sierra already installed to function.
softwareupdate --fetch-full-installer --full-installer-version 10.13.6 download macos high sierra 1013 6 dmg file link
If you already have an earlier version of High Sierra (10.13.x) and want to update to 10.13.6, you can download the macOS High Sierra 10.13.6 Combo Update DMG Standard Update (DMG): For users already on version 10.13.5, the macOS High Sierra 10.13.6 Update DMG is a smaller file. Apple Support Alternative Download Methods Apple hosts manual "Combo Update" files (like the 10
: For users specifically on 10.13.5. Download from Apple Support . 3. Alternative Tools for Modern Macs download macos high sierra 1013 6 dmg file link