Many software systems are established using client-server models. If a program on the server needs to be updated due to software maintenance, the service provided by the program will be required to terminate. The clients cannot be served until the update process is completed. The update process obviously reduces commercial profit especially for e-commerce systems. Therefore, a nonstop software update scheme is needed for high system availability. This paper develops an NSU system to update general server programs without stopping services. The NSU system can take over the task of old version using a redundant server for updating. The system supports applications with shared variables and maintains data consistency during software update. The current implementation is designed for Java programs running on the Linux operating system. Experimental results show that the NSU system incurred less than 1% overhead during normal execution.