I'm trying to update a 10 year old iMac currently running OS 10.10.5. The hardware appears to be in great shape.
When I go to Software Updates, it say no updates available.
When I manually download 10.11 or 10.12, the installer PKG will launch, go through some prompts, then say it's installed a few megabytes (MB) and stop without asking for a reboot. After a manual reboot, it's still running 10.10.5
When I go to App Store and try to install 10.13, it says there's a problem with the installer file.