Ico smart contract update

I would like to update this code, because it is CDT 1.2.
I would like to update for CDT 1.5 or CDT 1.6.
What do I need to change?

#include <eosiolib/eosio.hpp>
#include <eosiolib/currency.hpp>

using namespace eosio;

class hello : public eosio::contract {
  public:
      using contract::contract;

      hello(account_name self) : contract(self) {}

      /// @abi table 
      struct account {
          account_name owner;
          uint64_t balance;

          uint64_t primary_key() const { return owner; };
      };

      typedef eosio::multi_index<N(account), account> accs; 

      /// @abi action 
      void addbalance(account_name owner, uint64_t amount) {
           accs accounts(_self, owner);
           auto existing = accounts.find(owner);

           if (existing == accounts.end())
           {
               /* Add new account */
               accounts.emplace(_self, [&](auto& a) {
                   a.owner = owner;
                   a.balance = amount;
               });
           } else {
               /* Update existing account */
               const auto& st = *existing;
               accounts.modify(st, 0, [&](auto& a) {
                   a.balance += amount;
               });
           }
      }

      void on(const currency::transfer& t) {
          if (t.to == _self) {
              addbalance(t.from, t.quantity.amount);
          }
      }

      void use(account_name contract, account_name act) {
          if (act == N(transfer)) {
              on(unpack_action_data<currency::transfer>());
              return;
          }

          auto& thiscontract = *this;
          switch( act ) {
             EOSIO_API( hello, (addbalance));
          };
      }
};

extern "C" {
   [[noreturn]] void apply( uint64_t receiver, uint64_t code, uint64_t action ) {
      hello  h( receiver );
      h.use( code, action );
      eosio_exit(0);
   }
}