Open all windows. According to WebMD.com, the most common way to reduce radon levels is to vent out the home by allowing air from outside to enter and mix with the radon gas.
Install wall fans or use box fans in several rooms of your home. With the windows open, turn the fans on to facilitate airflow and to further vent out your home.
Seal all gaps around utility pipes and cracks in your basement floor with caulk. Once the home has been properly ventilated, seal off gaps and cracks. Radon commonly seeps in around these areas. Sealing these areas will prevent radon levels from increasing after you ventilate the house.
Hire a qualified radon contractor in your area and have her install radon-reduction systems in your home. For more complicated fixes, you should hire a radon contractor. The contractor can install such systems as soil radon reduction systems and subslab suction systems that further facilitate airflow and ventilation and reduce radon levels. See the link in Resources for a state radon office near you.