Once a patent has been filed and approved by USPTO, can it amended, changed, added or modified afterwards? To what extent of the contents can it be modified? Until what time interval (the deadline) of which it can still be modified (one year after approved? two years?)? Does this mean if someone found an invention and patent it, and when later he found some improvements to his invention he can add this improvement to his existing patent? If yes, how can he do so (what are the processes)? Please tell me whatever details you find relevant.