Every pepper is different and the heat levels will vary in your jalapeno syrup. To make it spicier you can add more jalapeno slices or use a different type of hot pepper (serranos, habaneros, etc.). To make it less spicy you can use less jalapenos, steep/simmer the syrup for less time, or remove them from the liquid before storing in the refrigerator. I don't recommend reducing the spice because once you add it to your drink you will likely lose the jalapeno flavor all together. Remember it will balance out once it has been mixed in with other liquids.