Examine your lighting plan and make adjustments for the CFL lights. You may need to add more fixtures to compensate for the decreased light output. HPS lights produce up to 140 lumens per watt, while CFL lights produce closer to 39 lumens per watt, so you need four CFL lamps for every one HPS lamp.
Purchase the appropriate compact fluorescent light bulbs for your application. For example, if you use them as grow lights, red spectrum work best for flowering, while blue spectrum work best for the leafing phase. Because HPS bulbs produce a yellowish color, you should generally replace them with red spectrum CFL bulbs.
Implement your plan. Remove the HPS light fixtures and install CFL light fixtures and bulbs; CFL bulbs do not fit into HPS fixtures. Install a reflective hood or lamp shade over grow lights to direct all of the light toward your plants, and lower the lamps towards your plants but do not let them touch the leaves--though CFL light bulbs do not produce as much heat, letting them touch may burn leaves.
Recycle your HPS light bulbs. They contain mercury, which poses an environmental hazard in a traditional landfill or incinerator, so use a reputable recycling facility that can handle mercury-containing lights.