Data centers have been widely deployed all over the world to meet demands for IT services, cloud computing, and IoT applications. Small data centers with insufficient cooling require much power to keep IT equipment running reliably. As a result, retrofitting has become an efficient method to increase energy efficiency and save operating costs. This study examines the impact of retrofitting on energy efficiency in a data center at a University in Taiwan. The data center under consideration underwent a comprehensive retrofitting process. Adding more effective cooling systems increases the data center's cooling capacity, resulting in more efficient cooling and improved temperature control, and eliminating an ineffective cooling system that helps to achieve an energy-efficient and cost-effective data center. A SCADA system was set up throughout the retrofitting procedure, which allowed a real-time study of energy consumption to identify and address any inefficiencies. Data were collected before and after retrofitting, including power usage effectiveness (PUE), energy consumption, and temperature measurements. Computational fluid dynamics (CFD) simulations were also conducted to determine the characteristics of the data center's airflow distribution and temperature profile. Numerical simulations conducted for two working scenarios have shown that making a small adjustment could significantly improve the airflow and temperature distribution. Numerical simulations have revealed that the development of hotspots at the top of the racks increased the air temperature beyond the allowable maximum temperature, which can be improved by adding effective cooling systems after retrofitting. The PUE of the data center before/after retrofitting was 2.17 and 1.41, respectively. The results showed a significant improvement in energy efficiency, with a PUE reduction of 0.76, which also reveals that energy efficiency has been improved by 35.02%. The retrofitting project results in significant cost savings and environmental benefits.