To install kernel 6.8 run in terminal
sudo apt install linux-generic linux-headers-generic
After the installation reboot and in grub menu select "Advanced options for Zorin" and select kernel 6.8 (not the recovery one) to boot from.
If you always want to boot with the 6.8 kernel, boot with the 6.8 kernel and then set it as default as described in this guide.
After each kernel update go to advanced options and select the latest version of the 6.8 kernel (or remove the hwe stack entirely from your system if you don't want to use higher kernels in future - then you don't need to care after kernel updates to set the latest 6.8 kernel).
If you want to remove the hwe stack, tell us, then we can give you the commands.