i can't get my apple to update the security update 2020

There was a security update released for macOS High Sierra and macOS Mojave at the end of May

beginning of June.

You have posted in the High Sierra community, but your profile says Mojave.

So which is it.

Anyway ignore the update through the App Store in High Sierra, if you are running Mojave

ignore the update in System Preferences> Software Update.

Try the standalone update that is right for the OS you are running.

High Sierra

https://support.apple.com/kb/DL2041?viewlocale=en_US&locale=en_US

Mojave

https://support.apple.com/kb/DL2042?viewlocale=en_US&locale=en_US

Whchever one is the correct one for your system, once downloaded double-click to install.

If you get a corrupted message...

Naw, not damaged, it's just that you have an old copy already in Applications, there are 2 cures...

Disconnect from the Internet & set the Date back to 2014, then you can run the Install MacsOS High Sierra App in Applications, so expired Certificates are not expired.

Or... trash the Install MacsOS High Sierra.app in Applications then a new copy more recent version can be downloaded.

Naw, not damaged, it's just that you have an old copy already in Applications, there are 2 cures...

Disconnect from the Internet & set the Date back to 2015, then you can run the Install MacsOS Mohave App in Applications, so expired Certificates are not expired.

Or... trash the Install MacsOS Mojave.app in Applications then a new copy can be downloaded.