Examine your reverse osmosis system. Check to see how many filters it has. The filters might be a screw on cylinder, where the entire cylinder is replaced, such as the system pictured with this article. Other systems might have a cylinder case, which unscrews, and a filter is placed inside.
Read the names of the different filters on the system. The filters might be named: Prefilter Cartridge, Reverse Osmosis System and Post Filter Cartridge. Or, they might be named Carbon, Carbon and Prefilter. The different systems will have different names for cartridges. Pay attention to the name of the filter, for when you replace a filter, you will need to insert it at the proper cartridge location.
Read the instruction book to your specific system to see how frequently the filters need to be changed. All the filters on a system are not typically changed at the same time. For example, one of the three filters might be changed every three months, while the second needs to be changed every six months, and the third needs to be changed once a year.
Unscrew the filters or cartridge container to change the necessary filter. If the cartridge and filter is one piece (such as the pictured system) then the entire cartridge will be unscrewed and replaced. If the cartridge and filter are two pieces, the cartridge will be unscrewed, the old filter will be removed, and the new filter will be slipped inside. The cartridge cover will then be screwed back onto the system.