First, you need to check whether your reference electrode is refillable or an old sealed model. A refillable reference electrode has two small holes in the plug at the surface where the electrical pin is installed. We can only suggest buying a new one if there are no holes.
Otherwise, please follow the procedure below. Apply all safety measures, including gloves, eye protection, and a lab coat. Refilling should be done when the solution level inside the reference electrode is about 25% of its full capacity. The procedure is straightforward. Insert one needle into one hole in the cap, and insert the other needle, attached to an electrolyte-filled syringe, into the other hole. Both needles need to pass through the septum, and you will feel resistance as you push through. Then, press the plunger on the syringe until the electrode is almost full. Be careful to avoid splashing electrolyte. Some are dangerous to the eyes and can stain severely.
You can watch a video tutorial here: How to Fill a Refillable Reference Electrode