GRUB_DEFAULT="Zorin, with Linux 6.8.0-52-generic"
Tap ctl+o to overwrite, then enter to save current configuration, then ctl+x to exit the editor.
Now, run
sudo update-grub
Reboot and test if it boots to 6.8
Alternatively
You can use this simple technique to set it to the Later kernel, by following the same steps, but doing so booted into the 6.8 kernel: