Upgrading OS Mac

I don't think there is a 12.15. Do you mean 12.1.5?

If you can get the installer for that version, you can install it.

To see available installers, use this command in Terminal:

softwareupdate --list-full-installers

To download a particular installer:

softwareupdate --fetch-full-installer --full-installer-version 12.1.5

If you are currently running version 12.1 you should be able to update to any available newer updates for the same OS, assuming that your computer will accept it. Sometimes they don't. I assume that it is due to an issue for specific models, etc. An upgrade is when a newer operating system is installed, such as upgrading from Ventura toSonoma.Then, what can be installed depends on the Mac's year and model.