First, you need to check whether your reference electrode is refillable or it is an old sealed model. The refillable reference electrode has two small holes in the plug at the surface where the electric pin is installed. We can only suggest buying a new one if there are no holes.
Otherwise, please follow the below procedure. Apply all the safety measures, including gloves, eye protection, and a labcoat. Re-filling should be done when the solution level inside the reference electrode is around 25% of the filled one. The procedure is straightforward. You need to place one needle in one hole in the cap while placing the other needle with an electrolyte-filled syringe in the other hole. Both needles need to go through the septum, and you will be able to feel it as it will resist pulling differently when you pass through. Then, press the plunger in the syringe until the electrode is almost complete. Be careful with splashing electrolytes. Some are dangerous to the eyes and stain severely.