I have created my token and deployed on ethereum network before ETH Fork Upgrades (E.g., ETH's Istanbul Upgrade), What will be the huge impact of Fork update on my smart contracts and how can i manage to update my smart contracts.
Most of the smart contracts are not upgradable by design, not sure about your one. When developers design hard fork, they take backward compatibility very seriously, so in general, existing smart contract should not be affected much. However, some behavior changes are possible. For example, Istanbul changes gas cost of certain operations (EIP-1884), so in case your smart contract uses any assumptions about gas costs of operations, it could be affected.
For example, if it calls other smart contracts and (explicitly or implicitly) limits amount of gas called contract is allowed to spend, then such calls may start running out of gas.
Another example is quite common pattern where smart contract obtains amount of gas left, and based on this value decides, whether to do another loop iteration or not. Such smart contract may stop working, because the minimum gas amount they believe is enough to gracefully finish transaction, could become not enough after Istanbul.
Could probably tell more if you will show your contract's source code.