How can metals be made better?

Answer 1

Heat treatment. Depending on the desired result. If "better" means higher toughness, it coul be annealed (Though it will be less hard) If you need higher hardness you could make a proper heat treatment such as tempering, solution hardening or maybe nitriding or carburizing.
If you need better corrosion resistance, well you can make a chrome, nickel or zinc plating. Another approach is to use a different alloy, which contains nickel and crhomium in convenient quantities.
Define what you mean with "better"

Answer 2

If you mean improving the properties depending on it's application, it can be done by Heat treatment....
Heat Treatment is the controlled heating and cooling of metals to alter their physical and chemical properties without changing the product shape...