Zorin OS 15
 

Does anyone know how to upgrade from Zorin OS 12.4 to Zorin OS 15?

I tried "apt update' then "apt dist-upgrade" but it didn't work.

According to the link an "in place" distribution upgrade tool is not available yet. Looks like your choices are wait or save your data and reinstall.

https://www.omgubuntu.co.uk/2019/06/zorin-os-15-linux

https://zorinos.com/help/upgrade-zorin-os/

Hopefully someone connected with Zorin will also reply.

Zorin 15 core
 

Have you tried to insert the disk to see if it offers to upgrade? I have seen a few Linux distros have it I have had a few distros that do it, as I know if you're on an older version and your updating it should already contain the 15 core files unless they process the option, I'm on the lastest 15 core I never upgraded, I did a fresh install on all my systems because I liked it overall out of 30 other distro builds. it does much better for me than I found in other ubuntu builds. more stable, ok I see how they want you to upgrade, heck that is easy basically you back up your system store it on either cd USB or external hard drive, then once 15 is installed you restore from backup.