Download new operating system

Start here: link-> How to reinstall macOS - Apple Support

Otherwise provide details, like model / year of computer, how you are trying to download new OS. What happens exactly when you try. Any error messages you receive etc...

You need to provide details if you want help.

Turn if off, and turn it back on with the Command (⌘)-R  keys held down to bring up Internet Recovery.

One options there should be to Re-Install macOS.

Disk Utility may also be there, but it's not the option you want.

See here: https://www.macworld.com/article/234851/how-to-fix-the-untrustedcerttitle-error-when-reinstalling-macos.html

That normally indicates an issue with the internal clock. Follow the steps in the link above to fix it.

Try this instead: https://mcgilldevtech.com/2019/05/macos-recovery-reinstall-untrusted-cert-title-error/