I want to deploy a smart contract, but I don't know which password to use. Is it the same as my main account on the real Ethereum network? And can I change the Testnet password? I forgot it, but I really want to see my smart contract in testnet ethereum.
Not necessarily, depends on you. Hopefully you are the only person knowing your password so if you forgot it, then there is no way to recover it. Now your wallet is encrypted and you'd really need your password to unlock your wallet. Maybe you can try this tool to crack your wallet.
You can only update your account password if you know your current password. More info here.
If you are not able to recover your password just create a new account and deploy the contract from that.