You do not need to refill the bottle during normal use, unless you notice that the water level has decreased (typically due to evaporation).
To refill the bottle with water, you can insert a feeding tube into one of the holes on the lid of the water bottle, and use an oral syringe to insert water into the bottle through the tube. Fill the bottle until you reach the desired pressure level.
You may need to add water to the bottle every day to maintain the same pressure level. This frequency will depend on the temperature and humidity of the room.