Upgrades are important. But that's got nothing to do with troubleshooting.
It seems that something has been broken by your customizations.
You didn't say where your template came from. Did you purchase it, or did you make it yourself? If you purchased it, have you any idea what Zen Cart version the template was designed for?
You also didn't indicate which Zen Cart version you are upgrading from, and what steps you have taken to do the upgrade.
I want to try it on the core side maybe to access the update cart function would be the best but i dont know how to do it. can you help me?I'm not sure what you mean by "core side"... hence my suggestion earlier to install a fresh copy of Zen Cart in another folder to test it uncustomized. If you let it install the demo products, that would let you add something to your cart, and do updates, etc, and see whether it works properly for you.
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