Assuming it is detached from hose, I would first unscrew the skinny end of the filter housing that goes into the gun at the gun handle. That's cause it looks fragile and allows easier access to the wrench positions where you break down the hex fitting from the filter housing. After unscrewing the hex fitting the filter cartridge should pull out easily for cleaning or replacement.
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Assuming it is detached from hose, I would first unscrew the skinny end of the filter housing that goes into the gun at the gun handle. That's cause it looks fragile and allows easier access to the wrench positions where you break down the hex fitting from the filter housing. After unscrewing the hex fitting the filter cartridge should pull out easily for cleaning or replacement.