macOS High Sierra 10.13.6 Combo Update (recommended for updating from any previous version of 10.13).
High Sierra 10.13.6 runs on Macs from late 2009 to 2017. It is the last version of macOS that supports certain legacy applications (32-bit apps) and older file systems (HFS+). download mac os high sierra 10136 dmg verified
Some legitimate developers archive old macOS installers. Two generally trusted sources: macOS High Sierra 10
The date on your Mac is wrong. Apple’s certificate expired on October 24, 2019. Fix: Disconnect from the internet, set the date back via Terminal: Some legitimate developers archive old macOS installers
The safest and most "verified" way to download High Sierra is via Apple’s hidden support pages. Apple keeps the High Sierra installer available because it is the maximum OS for several older Mac models.
Open Safari on a compatible Mac and visit the How to download and install macOS page.