upgrading 10.12.6 to 10.13.6 without clean install

App store does not have any legacy method for downloading system updates.

I need to upgrade a late 2009 iMac running 10.12.6 to 10.13.6 without wiping disk for a clean install. Is this possible?

If so, where to find the software (since Apple doesn't care to provide any)?

Thanks for vectors!