Press esc or tab during boot (continuosly after pressing the power button) to show the grub menu.
Then in grub menu select "Advanced options for Zorin" and choose the kernel you want to boot from.
This setting is reset after each reboot.
When it works with kernel 6.18 then it is also fine and you can keep this kernel.
If you want to set an older kernel as default to boot use this guide:
You only need to follow this guide for older kernels as e.g. kernel 6.8, not for kernel 6.18.