Booting from LiveUSb is the same step you took when you installed Zorin OS:
If following the above guide, you can stop at the portion of "Installing Zorin OS". You want to select Try Zorin from the LiveUSB grub menu.
Once booted, you can launch Gparted either from app menu or alt+F2
In gparted, click on the partition you want to manage.
If it is greyed out, you may need to unmount it. Launch the file manager in the Try Zorin demo and in the left pane of the file manager, look for the Volume Size of that partition listed. Click on the icon on the far right of it on that line to eject.
With it now unmounted, proceed with the steps from above;