Am downloading core can i update to Ultimate?
After Core is installed then pay and upgrade
In order to use Ultimate, you must install the Zorin Ultimate OS, as you did with Core.
However, Swarfendor has written a good guide on how to re-install the OS with a back up of Home:
Another approach would be to set up Core, read Swarf’s list of differences between Core and Ultimate and select the apps you need to build your own personal Ultra-Core-Lite, most of which are in the repository or available through Synaptic. Not everybody will use all the extras in Ultimate. That is time consuming but more fun. If you are pushed for time just install Ultimate following Aravisian and Swarf’s guidelines.