Hi. Once I upload my contract code (it's not an upgradeable contract).... Is there *any way that as the owner of the contract I can overwrite the contract code ?

What do you mean exactly by "overwriting"? You can change the values of variables IF there is a function that you implemented to do so - but once deployed you can not alter the source code/bytecode of your contract.

Even if it is "upgradeable" (one contract pointing to another and someone can update that pointer) you are effectively deploying a new smart contract to a new address. You are not altering the old one

noted. thanks.

How can I ensure that once my mint() function is called once, it is never called again?

Add a boolean state variable (for example "isMinted"), require that the value is false in the function and then set the value to true and mint

Well that's the whole point of creating a smart contract. To make it non overwrittable😅

Ya, just needed a sanity check.