OS update for my new iMac

You can try Download macOS Mojave 10.14.5 Combo Update - Apple Support https://support.apple.com/kb/DL2001

Once it is download reboot into Safe boot mode,

SafeBoot https://support.apple.com/en-us/HT201262

Takes noticeable longer to get to the login screen, does a 5-15 minute disk repair before it fully boots up, and certain system caches get cleared and rebuilt, including dynamic loader cache, etc. Log in then launch the installer.

menu : About This Mac. What version is currently installed?

If it is a 2019 iMac, it should have come with Mojave 10.14.5. Ensure that your Internet connection is working, and then in System Preferences : Software Update, let it inform you that the 10.14.5 update is available. If the machine is on 10.14.4, it may take a few seconds for software update to find and report about 10.14.5. Be patient through the several reboots.