My diagnosis and solution to a storage location issue problem in Solidity 101.

MemorySolution.sol

pragma solidity 0.5.1;
contract MemorySolution {

    mapping(uint => User) users;

    struct User{
        uint id;
        uint balance;
    }

    function addUser(uint id, uint balance) public {
        users[id] = User(id, balance);
    }

    function updateBalance(uint id, uint balance) public {
        // Problem: update is executing but no affect on balance
        User memory user = users[id];  // Copies mapped user to a memory User struct
        user.balance = balance; // Updates balance of memory User struct.
        
        // fails to push memory User struct to mapping in storage.
        users[id] = user; // <-- should fix it, if overwrites are allowed.
        
        // Note: User.id is superfluous to users[id]
    }

    function getBalance(uint id) view public returns (uint) {
        return users[id].balance;
    }

}