How to change a RO membrane
Some areas have softer water, while others quite the opposite. Source water that measures less than 300 TDS is best; anything higher is harder on the system. Measuring the source water's TDS levels can provide some idea of the conditions you are dealing with. The higher the TDS, the harder the membrane has to work, and decreases its life. Lower TDS-laden water is always better. Keep in mind that those numbers may vary throughout the year, affecting the way your RO system works.
1) Turn off the water going into the RO/DI system.