I install HVAC systems for side income. There's a chance your system is undersized for the home, but since it's what you've got you can make a few abnormal adjustments. Close All of your downstairs registers, you can partially open some up later if you have hot areas downstairs. Then, check for air returns upstairs, if there are a few, block off the downstairs returns, saran wrap works well.
This is assuming that the upstairs and downstairs are similar in square footage. You wouldn't have to do this on a correctly designed system, but it's easy and common.
Hope this helps,
Shaun