Depends on if you have a cartomizer, or a cartridge with a separate atomizer, you could get tweezers and pull out the filling if its simply a cartridge and fill the cartridge with the nicotine and replace the filling or you can simply drip the liquid on the filling until its saturated but you don't get as much liquid in it if you do it that way, a cartomizer is a cart and atomizer in one and needs a syringe, hope that helps.