I have central air in my condo. It only has one thermastat which is on the lower level. The lower level stays cool 70, but the upstairs is always too warm 75 -78+. How can I cool both upstairs and downstairs.

The problem is with the duct work for the system. This problem is well known in the residential construction industry. There is no real cure unless you consider tearing out your walls etc to add more duct going to the upstairs.

I suggest you contact a local contractor for a ON SITE inspection and recommendations.

Could always ad a second a/c for upstairs,, a window unit,a through the wall unit,, or a ductless system,,