I am not sure if your attempt to direct update has harmed your personal files in /home partition. If not then you can use the "Something Else" method to re-install Z15.3 Core, or if you wish Z16 Core if you have a bootable USB containing that.
I always use Wolfman's (old but good) guide to using the "Something Else" method to install ZorinOS. Here is link to it on the old Forum archive. Install Ubuntu (Zorin OS) using the "Something Else" method - Old Zorin Forum archive