Suck out the liquid from your cartridge with a syringe or dropper top and you can put it back in later (or roll in a paper towel to remove liquid). Remove the polyfill and pull the polyfill apart a bit with two straight pins. Reload it, and refill with your liquid. The polyfill can get squished down from your filler bottle top or from the atomizer bridge.
New stuff is cheap (Walmart, Target, Michael's Craft store or sewing shops) and you can
buy it in a big bag, or (my preference) get the sheet batting (which is made for quilting).