From the boot screen (Your motherboard splash screen), you can tap esc or tab (you may need a few trial and error sessions) if using EFI to reach the grub menu.
If you are using Legacy (MBR), instead hold down the Left Shift Key to pull up the grub menu.
The grub menu appears automatically if you are dual booting.
On it, you see the options to choose to boot Zorin OS or Advanced Options for Zorin~ and another OS if you have one installed. Zorin. Selecting Advanced Options...`, you will see the option to boot on a selected kernel or Zorin OS (Recovery).
Using arrow keys, move the selection down to select Zorin OS (Recovery)
From here, you will see the first option is Zorin on 7.0.xxx kernel. The one below that is Zorin on 7.0.xxx (recovery) and below that should be Zorin on 6.17.xxx...
So, tapping down twice on the arrow key should get you to Zorin on 6.17. Hit the enter key to select it to boot from.
If all is working, you can also set it to your default boot kernel if you want to.