4 stage RO/DI needs changing but which filter(s) ??

[kien]

So my 4 stage RO/DI unit was reading zero on the (handheld) TDS up until about a couple of weeks ago where it started reading 5ppm. I wasn't concerned yet, but last night it read 25ppm. I think now I need to look into replacing some filters but I have to admit I am a bit of a newbie with these RO/DI units. How do I tell which ones need replacing ? Or do I just replace them all? The whole unit is about 9 or 10 months old so that's how old all the filters are.