Welcome to Just Answer!. The problem you are having is common to two story homes where it is difficult to run large enough duct work to the upstairs..
...that happens because the architect has not designed space to run larger ducts....it is the fault of both the architect and the HVAC contractor.The solution is to find space in the back side of a closet or by some other means run a much larger air duct to the upstairs area... its not an easy project in most cases.Then after that is done, fit a separate thermostat in the upstairs to control air flow in the larger duct to control the temperature in the upstairs.Any *good HVAC contractor can do that work for you...costs will be in the $3,000 to $5,000 range as a rule.I hold questions open after positive ratings to allow for unlimited follow up IF you rate my answers positively. Please be patient though. The research & typing take timePhil