If it is the membrane itself, you repace the entire membrane - outer shell and all. I don't know that they actually open up on any unit.
Is that your question?
If you are asking about changing a cartridge out, then yes, the wrench that is usually supplied comes in very handy. If you don't have one, putting on a pair of rubber gloves often gives the boost you need to get it open..sometimes
Yes the membrane outer casing does come off. It is very tight. You will need to remove the pressure fittings and take out the housing from the bracket. That will allow better access and more torque. Make sure to label where everything came from before you take it apart so you can put it back together properly. or see below for tech support.
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Depends on your usage. If you change the filters and your tds meter is not reading 0 then it is time to change. Also it puts out less water than rated it is time to change.