Here is a Solidity code for the **New contract** that can call the f1() function from your **Old contract**. Hope this helps. I tried to keep it as simple as possible. 





        import "./Old.sol";    // You import the existing contract. 
            
            
                /**
                * The NEW contract will call the Old one
                */
            
              contract New {
              
                   Old OLD; // Intitilize old contract variable (empty)
            
                /**
                 * Set the address for Old contract (We call this function and enter the address of the OLD contract)
                 */
                function setOldContractAddress(address addr) public {
                    OLD = Old(addr);
                }
            
            
                 /**
                 * Function that allows us to call f1() from the Old contract
                 */
                function callOLDcontract() public {
                    OLD.f1();
                 
                    }
                
                }