The easiest way to fix the problem is by using the little lever on the registers to control air flow. For rooms that are too cold, close the register about 75% or all the way (i.e. closing it only halfway doesn’t make enough of an impact). This restriction in air flow will not only keep the room from being cooled, but also send more cold air to other rooms that need it.
Another way of controlling air flow is through opening and shutting duct dampers. These devices are built inline with the duct work of the central air system. They are easy to identify by the little handle on the side (i.e. not all homes have dampers). By closing a damper air flow will be restricted from an entire area of the house. To verify which registers were shut off, simply feel for air flow when the central air blower kicks on.
Be sure to be cautious when closing dampers. At least one must remain open at all times. It is best to try and solve the problem with registers first.