There are a lot of variables, so this may be something for which you may need to provide additional details, and may need the assistance of one of the HVAC geru's, but, in layman terms, heat rises, so I always like to start with the basics of throttling back some or all of the first floor registers, in order to force more cool air to the second floor. Some registers have built in throttles, but if not, you can experiment by covering or partially covering some of your first floor registers. Just to help you get some additional assistance though, how old is your home? Was it originally equipped with air conditioning? If it was retrofitted, it is not uncommon for the second floor to have inadequate ducting, due to the logistics involved.